WO2020142481A1

WO2020142481A1 - Quantum dot light-emitting diodes comprising electron spreading layer and fabrication method thereof

Info

Publication number: WO2020142481A1
Application number: PCT/US2019/069050
Authority: WO
Inventors: Paul H. Holloway; Baek Hyun KIM; Alexandre TITOV; Krishna ACHARYA
Original assignee: Nanophotonica, Inc.
Priority date: 2018-12-31
Filing date: 2019-12-31
Publication date: 2020-07-09
Also published as: TW202032810A

Abstract

Articles and methods related to quantum dot light-emitting diodes are generally provided. A quantum dot light-emitting diode may comprise a first electrode, a quantum dot light-emitting layer disposed on the first electrode, an electron transport layer disposed on the quantum dot light-emitting layer, an electron spreading layer disposed on the electron transport layer, and second electrode disposed on the electron spreading layer.

Description

Quantum Dot Light- Emitting Diodes Comprising Electron Spreading Layer and

Fabrication Method Thereof

FIELD

The present invention relates generally to quantum dot light-emitting diodes, and, more particularly, to electron spreading layers therefor.

BACKGROUND

Quantum dot light-emitting diodes may be useful in thin-film display and general lighting applications. However, the external quantum efficiency of current quantum dot light- emitting diodes reaches a maximum value at a low current density and then droops remarkably as the current density increases. This may be due to electron overflow out of the quantum dots and/or poor hole injection into the quantum dots. Additionally, quantum dot light-emitting diodes can display reduced efficiency when operated at both lower current densities and high current densities. At high current densities, it is not uncommon for only a small fraction of excess carriers can recombine into photons, reducing efficiency. At low current densities, it is not uncommon for there to be insufficient carriers to recombine into photons, reducing both optical power and causing efficiency droop. In some cases, electron blocking layers may be included in quantum dot light-emitting diodes to attempt to address these drawbacks. However, current electron blocking layers prevent electron flow, which causes an increase in the magnitude of the electric field across the device. The energetic electrons generated by high electric fields may be undesirable because they may cause degradation of the quantum dot light-emitting diodes and/or enhance current leakage therein.

Furthermore, the efficiency of current quantum dot light-emitting diodes is often limited by their inability to emit ah of the light that is generated by the quantum dot light-emitting layers therein. The light emitted by the quantum dot light-emitting layer often reaches the emitting surface at many different angles. Light that reaches the emitting surface above the critical angle will not pass therethrough but will instead experience undesirable total internal reflection. In some cases, light that undergoes total internal reflection can continue to be reflected within the quantum dot light-emitting diode until it is absorbed. Absorbed light may be transformed to thermal energy, disadvantageously heating the quantum dot light- emitting diode and possibly causing it to fail. Absorption of a portion of the light emitted by a quantum dot light-emitting diode therein also means that that light is not emitted therefrom, reducing the efficiency of the quantum dot light-emitting diode.

Therefore, there is a need for quantum dot light-emitting diodes having improved brightness and efficiency. There is also a need for quantum dot light-emitting diodes exhibiting higher current blocking, enhance current spreading, and increased light-extraction.

SUMMARY OF THE INVENTION

Electron spreading layers, related components, and related methods are generally described.

In some embodiments, a quantum dot light-emitting diode is provided. The quantum dot light-emitting diode comprises a first electrode, a quantum dot light-emitting layer disposed on the first electrode, an electron transport layer disposed on the quantum dot light-emitting layer, an electron spreading layer disposed on the electron transport layer, and a second electrode disposed on the electron spreading layer.

Other advantages and novel features of the present invention will become apparent from the following detailed description of various non-limiting embodiments of the invention when considered in conjunction with the accompanying figures. In cases where the present specification and a document incorporated by reference include conflicting and/or inconsistent disclosure, the present specification shall control. If two or more documents incorporated by reference include conflicting and/or inconsistent disclosure with respect to each other, then the document having the later effective date shall control.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present teachings in any way. In the drawings:

FIG. 1 is a perspective view illustrating a structure of a quantum dot light-emitting diode according to some embodiments;

FIG. 2 is a perspective view illustrating a structure of a quantum dot light-emitting diode according to some embodiments;

FIG. 3a is a perspective view illustrating an energy band structure of a quantum dot light- emitting diode according to some embodiments;

FIG. 3b is perspective view illustrating an energy band structure of a quantum dot light- emitting diode according some embodiments;

FIG. 4a is a perspective view illustrating a structure of a quantum dot light- emitting diode according to some embodiments;

FIG. 4b is a perspective view illustrating a structure of a quantum dot light-emitting diode according to some embodiments;

FIG. 5a is a graph showing the current density as a function of voltage for the quantum dot light-emitting diode described in Example 1 ;

FIG. 5b is a graph showing the external quantum efficiency as a function voltage for the quantum dot light-emitting diode described in Example 1 ;

FIG. 5c is a graph showing the external quantum efficiency as a function of luminance for the quantum dot light-emitting diode described in Example 1 ;

FIG. 6a is a graph showing the current density as a function of voltage for the quantum dot light-emitting diode described in Example 2;

FIG. 6b is a graph showing the external quantum efficiency as a function of voltage for the quantum dot light-emitting diode described in Example 2;

FIG. 6c is a graph showing the external quantum efficiency as a function of luminance for the quantum dot light-emitting diode described in Example 2; and

FIG. 6d is a graph showing the external quantum efficiency as a function of current density for the quantum dot light-emitting diode described in Example 2.

DETAILED DESCRIPTION

The present disclosure generally relates to electron spreading layers for quantum dot light- emitting diodes, and associated articles and methods. An electron spreading layer may be positioned between a cathode, from which electrons are injected into the quantum dot light- emitting diode, and an electron transport layer, through which electrons are transported to the light emitting layer. Some electron spreading layers described herein may advantageously distribute the electrons injected therein such that they are transported into the electron transport layer in an advantageous manner. For instance, an electron spreading layer may form an interface with a cathode to which it is adjacent that results in transport of electrons from the cathode to the electron spreading layer across a Schottky barrier that is relatively constant across the interface. This may advantageously promote a flow of current laterally across the electron spreading layer and then promote a flow of current across the interface in a manner that is relatively constant over the area of the interface. In other words, some electron spreading layers may cause electrons injected from a cathode (possibly in a non- uniform manner) to be introduced into the electron transport layer at a relatively constant areal density. As another example, an electron spreading layer may have a morphology that causes a relatively high percentage of the light emitted by the light emitting layer to escape the quantum dot light-emitting diode, advantageously reducing the light reabsorbed by the quantum dot light-emitting diode and increasing the efficiency of the light-emitting diode.

The quantum dot light-emitting diodes described herein may be suitable for use in a wide variety of applications, such as flat panel TV screens, digital cameras, mobile phones, AR/VR displays, Li-Fi communications, lighting, and handheld game consoles.

FIG. 1 shows a schematic view of a quantum dot light-diode according to one embodiment.

In FIG. 1, the quantum dot light-emitting diode comprises a substrate 210, an anode 220 disposed on the substrate, a hole injection layer 230 disposed on the anode, a hole transport layer 240 disposed on the hole injection layer, a quantum dot light-emitting layer 250 disposed on the hole transport layer, an electron transport layer 260 disposed on the quantum dot light-emitting layer, an electron spreading layer 280 disposed on the electron transport layer, and a cathode 270 disposed on the electron spreading layer. As used herein, when a layer is referred to as being“disposed on” another layer, it can be directly disposed on the layer, or an intervening layer also may be present. A layer that is“directly disposed on another layer means that no intervening layer is present. Accordingly, it should be understood that some quantum dot light-emitting diodes may include further layers not shown in FIG. 1 and/or positioned between two layers shown in FIG. 1. By way of example, in some embodiments a quantum dot light-emitting diode that comprises two hole injection layers and/or two hole transport layers. In the case of, for instance, a quantum dot light- emitting diode comprising two hole injection layers, a second hole injection layer may be positioned between the first hole injection layer and the hole transport layer. Similarly, quantum dot light-emitting diode comprising two hole transport layers may comprise a second hole transport layer positioned between the first hole transport layer and the quantum dot light-emitting layer.

It should be understood that, when a quantum dot light-emitting diode comprises two or more layers of the same type (e.g., two or more hole injection layers, two or more hole transport layers), the two layers of the same type may be identical or may differ in one or more ways. For instance, a quantum dot light-emitting diode may comprise two or more layers of the same type that differ in chemical composition, doping level, band gap, morphology, thickness, and/or another manner.

When a voltage is applied between the two electrodes, the anode 220 may inject holes into the hole injection layer 230. The holes may then be transported through the hole transport layer 240. Application of a voltage may also cause the cathode 270 to inject electrons into the electron spreading layer 280. The electrons may then be transported through the electron transport layer 260. The injected holes and injected electrons may combine in the quantum dot light-emitting layer 250 (e.g., at one or more quantum dots therein) to form excitons. The excitons may be recombined to emit light.

Some embodiments relate to methods of forming quantum dot light-emitting diodes, such as the quantum dot light-emitting diode shown in FIG. 1, quantum dot light-emitting diodes comprising one or more of the layers shown in FIG. 1, and/or quantum dot light-emitting diodes comprising further layers not shown in FIG. 1. In some embodiments, a method comprises assembling together one or more layers to form a quantum dot light-emitting diode. The layers may be assembled together by depositing one layer on another to form the quantum dot light-emitting diode. The layers may be deposited in the order shown in FIG. 1 (i.e., an anode may be deposited on a substrate, a hole injection layer may be deposited on the anode, a hole transport layer may be deposited on the hole injection layer, a quantum dot light-emitting layer may be deposited on the hole transport layer, an electron transport layer may be deposited on the quantum dot light-emitting layer, an electron spreading layer may be deposited on the electron transport layer, and a cathode may be deposited on the electron spreading layer), in the reverse order (i.e., an electron spreading layer may be deposited on a cathode, an electron transport layer may be deposited on the electron spreading layer, a quantum dot light-emitting layer may be deposited on the electron transport layer, a hole transport layer may be deposited on the quantum dot light-emitting layer, a hole injection layer may be deposited on the hole transport layer, an anode may be deposited on the hole injection layer, and a substrate may be deposited on the anode), in either the order shown in FIG. 1 or the reverse order but omitting the formation of one of the layers shown therein, and/or in another suitable order. A variety of suitable forms of deposition may be employed, such as spin coating, vacuum deposition, printing, spraying, roll-to-roll coating, dip coating, stamping, and the like.

A variety of suitable substrates may be employed. In some embodiments, it may be advantageous for the substrate to have optical transparency, include one or more surfaces that are smooth, be capable of easy handling, and/or have excellent water repellency. In some embodiments, the substrate comprises glass and/or a polymer. Non-limiting examples of suitable polymeric substrates include polyethylene terephthalate substrates and polycarbonate substrates.

A variety of suitable anodes may be employed. In some embodiments, the anode comprises a metal and/or a ceramic. Non-limiting examples of suitable metals include nickel (Ni), platinum (Pt), gold (Au), silver (Ag), and iridium (Ir). Non-limiting examples of suitable ceramics include indium tin oxide (ITO) and indium zinc oxide (IZO).

In some embodiments, the hole injection layer includes a polymer and/or a ceramic. Non limiting examples of suitable types of polymers include poly(3,4- ethylenedioxythiolphene):polystyrene para- sulfonate (PEDOT:PSS) derivatives, poly(9- vinylcarbazole) (PVK), poly(methyl methacrylate) (PMMA), and/or polystyrene. Non limiting examples of suitable ceramics include oxides, nitrides, carbides, sulfides, halide salts, citrate salts, nitrite salts, phosphate salts, thiocyanide salts, bicarbonate salts, and sulfide salts. Non-limiting examples of suitable oxides include M0O₃, AI₂O₃, WO₃, V₂O₅, NiO, MgO, HfC , Ga203, Gd203, La203, S1O2, Zr02, Y2O3, Ta203, T1O2, and BaO. One example of a suitable nitride is S1₃N₄. One example of a suitable carbide is SiC. One example of a suitable sulfide is ZnS. Non-limiting examples of suitable anions for halide salts include iodide anions, bromide anions, chloride anions, and fluoride anions. Non-limiting examples of suitable cations for halide salts include copper cations (e.g., the halide salt may be Cul, CuBr, Cul, and/or CuCl), alkali cations (e.g., the halide salt may comprise a lithium cation, and/or may comprise LiF and/or LiCl), and alkaline earth metal cations (e.g., the halide salt may comprise a magnesium cation, and/or may comprise MgFi). When a quantum dot light-emitting diode comprises two or more hole injection layers, each hole injection layer may independently comprise one or more of the materials described above.

The hole transport layer may include a polymer, an organic molecule, and/or a ceramic.

Non-limiting examples of suitable polymers include poly[(9,9-dioctylfluorenyl-2,7-diyl)-co- (4,4'-(N-(4-sec-butylphenyl)) diphenylamine)] (TFB), PVK, poly(N,N’-bis(4-butylphenyl)-

N,N'-bis(phenyl)benzidine) (poly-TPD), poly-N-vinylcarbazole, polyphenylene- vinylene, polyparaphenylene, a polymethacrylate derivative, poly(9,9-octylfluorene), poly(spiro- fluorene), tris(3-methylphenylphenylamino) triphenylamine (m-MTDATA), poly[2-methoxy- 5-(2'-ethylhexyloxy)-l,4-phenylene vinylene] (MEH-PPV) and poly[2-methoxy-5-(3',7'- dimethyloctyloxy)-l,4-phenylene vinylene] (MDMO-PPV). Non-limiting examples of suitable organic molecules include TPD (N,N'-Bis(3-methylphenyl)-N,N'-diphenylbenzidine) and NPB (N,N'-Di(l-naphthyl)-N,N'-diphenyl-(l,l'-biphenyl)-4,4'-diamine). Non-limiting examples of suitable ceramics include copper (I) iodide (Cul), copper (I) thiocyanate

(CuSCN), copper gallium oxide (CuGaOi), and copper aluminum oxide (CuAlC ). When a quantum dot light-emitting diode comprises two or more hole transport layers, each hole transport layer may independently comprise one or more of the materials described above.

The hole transport layer may have a variety of suitable morphologies. In some

embodiments, the hole transport layer comprises one or more nanoparticles. Such nanoparticles may be crystalline, amorphous, or partially crystalline and partially amorphous. For instance, in some embodiments, the hole transport layer comprises nanocrystals. When the hole transport layer comprises nanoparticles, it may further comprise one or more ligands surrounding and/or passivating the nanoparticles. Non-limiting examples of suitable ligands include oleic acid, 1-hexadecanethiol, 1-octanethiol, 1-dodecanethiol, 1-hexanethiol, ethanethiol, butanethiol, 1-pentanethiol, 1-propanethiol, 1,2-ethanedithiol, 1,4-butanedithiol, 1,6-hexanedithiol, 1,8-octanedithiol, and 1,10-decanedithiol. When a quantum dot light- emitting diode comprises two or more hole transport layers comprising nanoparticles, each hole transport layer may independently comprise one or more of the ligands described above.

When the hole transport layer comprises nanoparticles, the nanoparticles may have a variety of suitable diameters. In some embodiments, the hole transport layer comprises

nanoparticles having a number average diameter of less than or equal to 200 nm, less than or equal to 150 nm, less than or equal to 100 nm, less than or equal to 75 nm, less than or equal to 50 nm, less than or equal to 25 nm, less than or equal to 10 nm, less than or equal to 5 nm, or less than or equal to 2 nm. The hole transport layer may comprise nanoparticles having a number average diameter of greater than or equal to 1 nm, greater than or equal to 2 nm, greater than or equal to 5 nm, greater than or equal to 10 nm, greater than or equal to 25 nm, greater than or equal to 50 nm, greater than or equal to 75 nm, greater than or equal to 100 nm, or greater than or equal to 150 nm. Combinations of the above-referenced ranges are also possible (e.g., less than or equal to 200 nm and greater than or equal to 1 nm). Other ranges are also possible. The number average diameter of the nanoparticles may be determined by electron microscopy. When a quantum dot light-emitting diode comprises two or more hole transport layers comprising nanoparticles, each hole transport layer may independently comprise nanoparticles having a number average diameter in one or more of the ranges described above.

In some embodiments, it is desirable for the hole transport layer to be relatively thin.

Without wishing to be bound by any particular theory, it is believed that thicker layers may exhibit enhanced carrier absorption loss in comparison to thinner layers. Free carrier absorption loss may undesirably cause light emitted by the quantum dot light-emitting layer to be reabsorbed in the hole transport layer instead of being emitted by the quantum dot light- emitting diode. The hole transport layer may have a thickness of less than or equal to 1 micron, less than or equal to 750 nm, less than or equal to 500 nm, less than or equal to 250 nm, less than or equal to 100 nm, less than or equal to 75 nm, less than or equal to 50 nm, less than or equal to 25 nm, less than or equal to 10 nm, less than or equal to 5 nm, or less than or equal to 2 nm. In some embodiments, the hole transport layer has a thickness of greater than or equal to 1 nm, greater than or equal to 2 nm, greater than or equal to 5 nm, greater than or equal to 10 nm, greater than or equal to 25 nm, greater than or equal to 50 nm, greater than or equal to 75 nm, greater than or equal to 100 nm, greater than or equal to 250 nm, greater than or equal to 500 nm, or greater than or equal to 750 nm. Combinations of the above-referenced ranges are also possible (e.g., less than or equal to 1 micron and greater than or equal to 1 nm). Other ranges are also possible. The thickness of the hole transport layer may be measured by electron microscopy. When a quantum dot light-emitting diode comprises two or more hole transport layers, each hole transport layer may independently have a thickness in one or more of the ranges described above.

The hole transport layer may be fabricated by a variety of suitable methods. In some embodiments, the hole transport layer is fabricated by a process comprising solution synthesis of nanocrystals. The nanocrystals may be formed into a layer by, e.g., spin coating, dipping and/or spraying onto a substrate and/or other layer of the quantum dot light- emitting diode. When a quantum dot light-emitting diode comprises two or more hole transport layers, each hole transport layer may independently be fabricated by one or more of the processes described above.

The quantum dot light-emitting layer may comprise a variety of suitable types of quantum dots. The quantum dots may be nanocrystalline, may be amorphous, or may be partially crystalline and partially amorphous. In some embodiments, the quantum dot light-emitting layer comprises a Group II- VI compound semiconductor quantum dot, such as a Group II-VI compound semiconductor nanocrystal quantum dot. Non-limiting examples of suitable Group II-VI compound semiconductor nanocrystal quantum dots include those comprising CdS, CdSe, ZnS, ZnSe, HgS, HgSe, HgTe, and/or alloys thereof. In some embodiments, the quantum dot light-emitting layer comprises a Group III-V compound semiconductor quantum dot, such as a Group III-V compound semiconductor nanocrystal quantum dot. Non limiting examples of suitable Group III-V compound semiconductor nanocrystal quantum dots include those comprising GaN. InN, AIN, GaP, GaAs, InP, GaSb, InSb, InAs, and/or alloys thereof. In some embodiments, the quantum dot light-emitting layer comprises a Group IV- VI compound semiconductor quantum dot, such as Group IV- VI compound semiconductor nanocrystal quantum dot. Non-limiting examples of suitable Group IV- VI compound semiconductor nanocrystal quantum dots include those comprising PbS, PbSe, PbTe, and/or alloys thereof.

The quantum dots may have a uniform composition, or may have a composition that varies spatially. For instance, in some embodiments, a quantum dot light-emitting layer comprises a core-shell quantum dot (e.g., CdSe/ZnS core/shell, CdS/ZnSe core/shell, InP/ZnS core/shell, and the like) The core-shell quantum dot may comprise a core with a first composition and a shell surrounding the core of a second, different composition. In some such embodiments, the material forming the core has a relatively small bandgap (e.g., CdSe, CdS, etc.) and the material forming the shell has a relatively large bandgap (e.g., ZnS, ZnSe, etc.). By way of example, a core-shell quantum dot may have a core comprising CdSe and/or CdS, and a shell comprising ZnS and/or ZnSe. Further examples of suitable core shell quantum dots are described in US 9,887,318, incorporated herein by reference in its entirety for all purposes.

Quantum dots employed in the quantum dot light-emitting layers described herein may have diameters that are in the range of nanometers to hundreds of nanometers. For instance, a quantum dot light-emitting layer may comprise quantum dots having a number average diameter of less than or equal to 200 nm, less than or equal to 150 nm, less than or equal to 100 nm, less than or equal to 75 nm, less than or equal to 50 nm, less than or equal to 25 nm, less than or equal to 10 nm, less than or equal to 5 nm, or less than or equal to 2 nm. The quantum dot light-emitting layer may comprise quantum dots having a number average diameter of greater than or equal to 1 nm, greater than or equal to 2 nm, greater than or equal to 5 nm, greater than or equal to 10 nm, greater than or equal to 25 nm, greater than or equal to 50 nm, greater than or equal to 75 nm, greater than or equal to 100 nm, or greater than or equal to 150 nm. Combinations of the above-referenced ranges are also possible (e.g., less than or equal to 200 nm and greater than or equal to 1 nm). Other ranges are also possible. The number average diameter of the quantum dots in a quantum dot light-emitting layer may be determined by electron microscopy.

Quantum dots employed in the quantum dot light-emitting layers described herein may emit light at a variety of suitable wavelengths. For instance, quantum dot light-emitting layers may comprise quantum dots that emit ultraviolet light, visible light, and/or infrared light. If the light is visible light, it may be a variety of suitable colors. By way of example, in some embodiments, a quantum dot light-emitting layer comprises quantum dots that emit red light, orange light, yellow light, green light, blue light, indigo light, and/or violet light. In some embodiments, a quantum dot light-emitting layer comprises quantum dots that emit light at a wavelength of greater than or equal to 100 nm, greater than or equal to 150 nm, greater than or equal to 200 nm, greater than or equal to 250 nm, greater than or equal to 300 nm, greater than or equal to 350 nm, greater than or equal to 400 nm, greater than or equal to 450 nm, greater than or equal to 500 nm, greater than or equal to 550 nm, greater than or equal to 600 nm, greater than or equal to 650 nm, greater than or equal to 700 nm, greater than or equal to 750 nm, greater than or equal to 800 nm, greater than or equal to 850 nm, greater than or equal to 900 nm, greater than or equal to 950 nm, greater than or equal to 1 micron, greater than or equal to 1.5 microns, greater than or equal to 2 microns, greater than or equal to 2.5 microns, greater than or equal to 3 microns, greater than or equal to 3.5 microns, greater than or equal to 4 microns, or greater than or equal to 4.5 microns. In some embodiments, a quantum dot light-emitting layer comprises quantum dots that emit light at a wavelength of less than or equal to 5 microns, less than or equal to 4.5 microns, less than or equal to 4 microns, less than or equal to 3.5 microns, less than or equal to 3 microns, less than or equal to 2.5 microns, less than or equal to 2 microns, less than or equal to 1.5 microns, less than or equal to 1 micron, less than or equal to 950 nm, less than or equal to 900 nm, less than or equal to 850 nm, less than or equal to 800 nm, less than or equal to 750 nm, less than or equal to 700 nm, less than or equal to 650 nm, less than or equal to 600 nm, less than or equal to 550 nm, less than or equal to 500 nm, less than or equal to 450 nm, less than or equal to 400 nm, less than or equal to 350 nm, less than or equal to 300 nm, less than or equal to 250 nm, less than or equal to 200 nm, or less than or equal to 150 nm. Combinations of the above- referenced ranges are also possible (e.g., greater than or equal to 100 nm and less than or equal to 5 microns). Other ranges are also possible. The wavelength(s) emitted by a quantum dot light-emitting layer may be determined by use of UV-vis-IR spectroscopy.

A variety of suitable materials may be employed in the electron transport layers described herein. In some embodiments, the electron transport layer comprises an oxide, such as T1O2, ZnO, S1O2, Ga203, AI2O3, MgO, Hf02, Zr02, and/or Ta203. In some embodiments, the electron transport layer comprises a nitride, such as S13N4, GaN, AIN, and/or TaN. In some embodiments, the electron transport layer comprises a carbide, such as SiC. In some embodiments, the electron transport layer comprises a semiconductor, such as CdS, ZnSe, and/or ZnS.

The electron transport layer may have a variety of suitable morphologies. In some embodiments, the electron transport layer comprises one or more nanoparticles. Such nanoparticles may be crystalline, amorphous, or partially crystalline and partially amorphous. When the electron transport layer comprises nanoparticles, the nanoparticles may have a variety of suitable diameters. In some embodiments, the electron transport layer comprises nanoparticles having a number average diameter of less than or equal to 200 nm, less than or equal to 150 nm, less than or equal to 100 nm, less than or equal to 75 nm, less than or equal to 50 nm, less than or equal to 25 nm, less than or equal to 10 nm, less than or equal to 5 nm, or less than or equal to 2 nm. The electron transport layer may comprise nanoparticles having a number average diameter of greater than or equal to 1 nm, greater than or equal to 2 nm, greater than or equal to 5 nm, greater than or equal to 10 nm, greater than or equal to 25 nm, greater than or equal to 50 nm, greater than or equal to 75 nm, greater than or equal to 100 nm, or greater than or equal to 150 nm. Combinations of the above-referenced ranges are also possible (e.g., less than or equal to 200 nm and greater than or equal to 1 nm).

Other ranges are also possible. The number average diameter of the nanoparticles may be determined by electron microscopy.

A variety of suitable materials may be employed in the cathodes described herein. In some embodiments, it may be advantageous for the cathode to have a relatively low work function, which may facilitate injection of electrons into the electron transport layer and/or into the electron injection layer. The work function of the cathode may be, for example, less than or equal to 4.8 eV, less than or equal to 4.6 eV, less than or equal to 4.4 eV, less than or equal to 4.2 eV, less than or equal to 3.9 eV, less than or equal to 3.7 eV, less than or equal to 3.5 eV, less than or equal to 3.2 eV, less than or equal to 3 eV, less than or equal to 2.8 eV, less than or equal to 2.6 eV, less than or equal to 2.4 eV, less than or equal to 2.2 eV, less than or equal to 2 eV, or less than or equal to 1.8 eV. The work function of the cathode may be greater than or equal to 1.5 eV, greater than or equal to 1.8 eV, greater than or equal to 2 eV, greater than or equal to 2.2 eV, greater than or equal to 2.4 eV, greater than or equal to 2.6 eV, greater than or equal to 2.8 eV, greater than or equal to 3 eV, greater than or equal to 3.2 eV, greater than or equal to 3.5 eV, greater than or equal to 3.7 eV, greater than or equal to 3.9 eV, greater than or equal to 4.2 eV, greater than or equal to 4.4 eV, or greater than or equal to 4.6 eV.

Combinations of the above-referenced ranges are also possible (e.g., less than or equal to 4.8 eV and greater than or equal to 1.5 eV, less than or equal to 3.9 eV and greater than or equal to 1.5 eV). Other ranges are also possible.

In some embodiments, the cathode comprises a metal and/or a metal alloy with a low work function, such as Ca, Cs, Ba, Al, Mg, Ag, and/or alloys thereof. In some embodiments, the cathode comprises an oxide with a low work function, such as ITO. Other examples of types of material that may be included in a cathode include alkali salts, halide salts, and alkali halide salts (e.g., LiF). The cathode may comprise a combination of two materials, at least one of which has a low work function, such as a combination of Ca and Al, a combination of LiF and Ca, and/or a combination of LiF and Al.

The quantum dot light-emitting diodes described herein may be encapsulated in a resin. For instance, some quantum dot light-emitting diodes described herein may be encapsulated in a UV-curable resin. In some embodiments, the resin comprises an unsaturated carboxylic acid (e.g., acrylic acid, methacrylic acid, benzoic acid, 3-butenoic acid, crotonic acid) and/or another suitable species that promotes positive aging of the encapsulated QD-LED. Non limiting examples of suitable resins include those described in U.S. Patent No. 9,780,256, incorporated herein by reference in its entirety for all purposes.

As described above, the quantum dot light-emitting diodes described herein may comprise an electron spreading layer. The electron spreading layer may comprise a variety of suitable materials. In some embodiments, the electron spreading layer comprises an inorganic compound, such as an oxide (e.g., a simple oxide; a complex oxide, such as an intermediate phase of a binary oxide), a nitride (e.g., a simple nitride, a complex nitride), an oxynitride, a carbide (e.g., a simple carbide, a complex carbide), a sulfide (e.g., a simple sulfide, a complex sulfide), a halide salt (e.g., a simple halide salt, a complex halide salt), a citrate salt (e.g., a simple citrate salt, a complex citrate salt), a nitrite salt (e.g., a simple nitrate salt, a complex nitrate salt), a phosphate salt (e.g., a simple phosphate salt, a complex phosphate salt), a thiocyanide salt (e.g., a simple thiocyanide salt, a complex thiocyanide salt), a bicarbonate salt (e.g., a simple bicarbonate salt, a complex bicarbonate salt), and/or a sulfate salt (e.g., a simple sulfate salt, a complex sulfate salt). Non-limiting examples of suitable oxides include AI2O3, MgO, HfC , Ga203, Gd2C>3, La203, Si02, Zr02, Y2O3, Ta203, ΉO2, and BaO. One example of a suitable nitride is Si3N4. One example of a suitable carbide is SiC. One example of a suitable sulfide is ZnS. Non-limiting examples of suitable anions for halide salts include iodide anions, bromide anions, chloride anions, and fluoride anions. Non limiting examples of suitable cations for halide salts include copper cations (e.g., the halide salt may be Cul, CuBr, Cul, and/or CuCl), alkali cations (e.g., the halide salt may comprise a lithium cation, and/or may comprise LiF and/or LiCl), and alkaline earth metal cations (e.g., the halide salt may comprise a magnesium cation, and/or may comprise MgF2). The electron spreading layer may comprise an organic material, such as a polymer (e.g., PVK, poly(methyl methacrylate) (PMMA), and/or polystyrene). The electron spreading layer may comprise two or more of the above-referenced materials. In some embodiments, a quantum dot light-emitting diode comprises two or more electron spreading layers disposed on each other. In such cases, each layer may have the same composition, or two or more electron spreading layers may have the same composition.

It should be noted that the electron spreading layer typically has a different composition than the electron transport layer. Some materials may be suitable for both the electron spreading layer and the electron transport layer, but, if employed in the electron spreading layer, should be used in combination with an electron transport layer having a different composition (or, if employed in the electron transport layer, should be used in combination with an electron spreading layer having a different composition). The compositional differences between the electron spreading layer and the electron transport layer may be relatively small. For instance, in some embodiments, a quantum dot light-emitting diode comprises an electron spreading layer that differs from an electron transport layer only in the type and/or amount of dopants therein. Typically, the electron spreading layer and the electron transport layer are selected such that the electron transport layer has a lower conduction band edge than the electron spreading layer and/or such that the Fermi level of the electron transport layer is lower than that of the electron spreading layer.

It is also noted that some materials that may be suitable for the electron spreading layer may also be suitable for the hole injection layer. Without wishing to be bound by theory, it is believed that materials of this type may both form an advantageous Schottky barrier with the cathode, making them suitable for use in the electron spreading layer, and have a conduction band that is similar in energy to or higher in energy than the conduction band of the quantum dots in a quantum dot layer, making them suitable for use in the hole injection layer.

The electron spreading layer may have a variety of suitable structures. For instance, a quantum dot light-emitting diode may comprise a continuous electron spreading layer, and/or may comprise a discontinuous electron spreading layer. The discontinuous electron spreading layer may comprise one or more portions that are not topologically connected to each other through the electron spreading layer. In some embodiments, the electron spreading layer does not fully cover the layer on which it is disposed (e.g., an electron transport layer, a cathode) and/or leaves one or more portions of the layer on which it is disposed exposed to a layer on the opposite side of the electron spreading layer (e.g., an electron transport layer, a cathode). In some embodiments, the discontinuous electron spreading layer comprises pores and/or openings. The electron spreading layer may be made up of, and/or comprise, particles, such as nanoparticles.

FIG. 2 shows a schematic view of a quantum dot light-diode comprising a discontinuous electron spreading layer. In FIG. 2, the quantum dot light-emitting diode comprises an anode 320 disposed on a substrate 310, a hole injection layer 330 disposed on the anode, a hole transport layer 340 disposed on the hole injection layer, a quantum dot light-emitting layer 350 disposed on the hole transport layer, an electron transport layer 360 disposed on the quantum dot light-emitting layer, an electron spreading layer 380 disposed on the electron transport layer, and a cathode 370 disposed on the electron spreading layer.

The electron spreading layers described herein may have a variety of suitable thicknesses. Without wishing to be bound by any particular theory, it is believed that thicknesses of less than 10 nm may be particularly desirable. It is believed that thicknesses below 10 nm facilitate injection of the electrons into the quantum dot light-emitting layer (and the quantum dots therein) and that thicknesses above 10 nm may block electron transport into the quantum dot light-emitting layer (and the quantum dots therein). In some embodiments, the thickness of the electron spreading layer is less than or equal to 10 nm, less than or equal to 7.5 nm, less than or equal to 5 nm, less than or equal to 2.5 nm, or less than or equal to 1 nm. In some embodiments, the thickness of the electron spreading layer is greater than or equal to 0.5 nm, greater than or equal to 1 nm, greater than or equal to 2.5 nm, or greater than or equal to 7.5 nm. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 0.5 nm and less than or equal to 10 nm). Other ranges are also possible. The thickness of the electron spreading layer may be measured by electron microscopy.

For an electron spreading layer that is continuous, the thickness referred to herein is the average thickness of the layer as a whole. For an electron spreading layer that is

discontinuous, the thickness referred to herein is the average thickness of the material forming the electron spreading layer (i.e., it does not include the zero thicknesses of any pores or openings passing straight through the layer or the zero thicknesses of any portions of the layer unoccupied by solid material).

The electron spreading layer may be formed on the electron transport layer. For instance, an appropriate material for the electron spreading layer may be selected and deposited on the electron transport layer to form a film. The appropriate material may be a precursor for the electron transport layer which may be capable of undergoing a reaction, such as a decomposition reaction, to form the electron transport layer. Non-limiting examples of suitable precursors include nitrate salts and acetate salts. The salt may further comprise a cation, such as an A1 cation, a Mg cation, a Hf cation, a Ga cation, a Gd cation, a La cation, a Si cation, a Zr cation, an Y cation, a Ta cation, a Ti cation, and/or a Ba cation. In some embodiments, the appropriate material is or comprises a material that forms the electron spreading layer without undergoing any further reactions (e.g., the appropriate material may be or comprise one or more of the materials described above as suitable for use in an electron spreading layer).

In some embodiments, one or more layers described herein, such as an electron transport layer, a quantum dot light-emitting layer, a hole transport layer, or another layer, may be deposited by a solution coating process. Non-limiting examples of suitable solution coating processes include sol-gel coating, spin coating, printing, casting, stamping, dip coating, roll- to-roll coating, and spraying. In some embodiments, solution coating processes may be desirable because they may be lower cost than other methods of forming thin films and/or may be performed at lower temperatures than other methods of forming thin films. The fluid employed during the solution coating process may be a dispersion comprising a precursor material as described above and a dispersion solvent. The dispersion solvent may comprise an aqueous solvent, such as water, and/or an organic solvent, such as an alcohol. Non-limiting examples of suitable alcohols include isopropyl alcohol, ethanol, methanol, butanol, pentanol, cetyl alcohol, and/or 2-methoxyl ethanol.

After deposition, the film can be annealed at from 70 °C to about 200 °C in air and/or oxygen.

Thermal annealing may comprise heating a deposited precursor to a temperature of greater than or equal to 50 °C, greater than or equal to 75 °C, greater than or equal to 100 °C, greater than or equal to 125 °C, greater than or equal to 150 °C, or greater than or equal to 175 °C. Thermal decomposition may comprise heating a deposited precursor to a temperature of less than or equal to 200 °C, less than or equal to 175 °C, less than or equal to 150 °C, less than or equal to 125 °C, less than or equal to 100 °C, or less than or equal to 75 °C. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 50 °C and less than or equal to 200 °C). Other ranges are also possible. Thermal annealing may comprise heating a deposited precursor to a temperature in one or more of the above-referenced ranges for a time period of greater than or equal to 1 min, greater than or equal to 2 min, greater than or equal to 5 min, greater than or equal to 10 min, greater than or equal to 20 min, greater than or equal to 30 min, greater than or equal to 1 hr, greater than or equal to 2 hr, greater than or equal to 5 hr, greater than or equal to 10 hr, greater than or equal to 20 hr, or greater than or equal to 50 hr. Thermal decomposition may comprise heating a deposited precursor to a temperature in one or more of the above- referenced ranges for a time period of less than or equal to 100 hr, less than or equal to 50 hr, less than or equal to 20 hr, less than or equal to 10 hr, less than or equal to 5 hr, less than or equal to 2 hr, less than or equal to 1 hr, less than or equal to 30 min, less than or equal to 20 min, less than or equal to 10 min, less than or equal to 5 min, or less than or equal to 2 min. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 1 min and less than or equal to 1 hr). Other ranges are also possible.

In some embodiments, a further treatment may be performed after annealing in order to decompose an initially deposited precursor to form the final, desired electron transport layer. For instance, one or more of an ozone treatment, an oxygen plasma treatment, and/or thermal decomposition can be employed. When a film is both annealed and then subsequently thermally decomposed, the thermal decomposition temperature is typically higher than the annealing temperature. The annealing temperature may be a temperature that promotes the formation of a desirable film morphology without chemically decomposing any of the materials therein. The thermal decomposition temperature may be a temperature that causes the materials therein to decompose. The selection of the annealing temperature and thermal decomposition temperature will depend on the chemistry of the precursor for the electron transport layer.

In some embodiments, at least a portion of a precursor material, such as a precursor material employed to form an electron spreading layer, may be thermally decomposed during the formation of an layer to form one or more pores and/or openings therein. The precursor material to be thermally decomposed to form the pores and/or openings may be positioned between other precursor materials (e.g., precursor materials not to be thermally decomposed, precursor materials to be thermally decomposed to form non-gaseous products), and may be decomposed such that all or a substantial portion of its volume is transformed into a gas.

The gas may escape, and the volume previously occupied by the thermally decomposed precursor may become a pore and/or opening. For instance, forming an electron spreading layer may comprise depositing polymeric spheres (e.g., polystyrene spheres), and then thermally decomposing the polymeric spheres to form carbon dioxide. The polystyrene spheres may be positioned between, for example, zinc oxide and alumina precursors. In some embodiments, the polystyrene spheres are embedded in alumina precursors.

Thermal decomposition may comprise heating a deposited precursor to a temperature of greater than or equal to 50 °C, greater than or equal to 75 °C, greater than or equal to 100 °C, greater than or equal to 125 °C, greater than or equal to 150 °C, or greater than or equal to 175 °C. Thermal decomposition may comprise heating a deposited precursor to a temperature of less than or equal to 200 °C, less than or equal to 175 °C, less than or equal to 150 °C, less than or equal to 125 °C, less than or equal to 100 °C, or less than or equal to 75 °C. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 50 °C and less than or equal to 200 °C). Other ranges are also possible.

Thermal decomposition may comprise heating a deposited precursor to a temperature in one or more of the above-referenced ranges for a time period of greater than or equal to 1 min, greater than or equal to 2 min, greater than or equal to 5 min, greater than or equal to 10 min, greater than or equal to 20 min, greater than or equal to 30 min, greater than or equal to 1 hr, greater than or equal to 2 hr, greater than or equal to 5 hr, greater than or equal to 10 hr, greater than or equal to 20 hr, or greater than or equal to 50 hr. Thermal decomposition may comprise heating a deposited precursor to a temperature in one or more of the above- referenced ranges for a time period of less than or equal to 100 hr, less than or equal to 50 hr, less than or equal to 20 hr, less than or equal to 10 hr, less than or equal to 5 hr, less than or equal to 2 hr, less than or equal to 1 hr, less than or equal to 30 min, less than or equal to 20 min, less than or equal to 10 min, less than or equal to 5 min, or less than or equal to 2 min. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 1 min and less than or equal to 1 hr). Other ranges are also possible.

Thermal decomposition as described herein may be carried out in the presence of air and/or oxygen.

In some embodiments, an electron spreading layer may have a design that is particularly beneficial. For instance, the electron spreading layer may have a design that results in a relatively constant Schottky barrier between it and the cathode. This may advantageously promote uniform current transport from the cathode into the electron spreading layer. The shape of the Schottky barrier (e.g., whether it is continuous or discontinuous, its magnitude) may be computed based on the known energy differences between the materials forming the Schottky barrier and their morphologies at the interface therebetween. For instance, FIG. 3a shows an inhomogeneous Schottky barrier of a quantum dot light-emitting diode. Without wishing to be bound by any particular theory, it is believed that the junction current of this inhomogeneous Schottky barrier can be described by a parallel conduction model. In such a model, the junction current is a linear sum of the contribution from every individual area, namely, 1(1 ⁼ A^*T² [exp( ?14)— 1 å_ϊAibcr(_bFi), where Ai and Fi are, respectively, the area and Schottky barrier height of the i-th“patch”. Here, the overall transport mechanism has been assumed to be thermionic emission. It is believed that the shape of the

inhomogeneous Schottky barrier height may have an effect (and/or a strong effect) on the current transport from the cathode into the electron spreading layer. The conduction across a region with a relatively low Schottky barrier height is“pinched-off’ by regions with relatively high Schottky barrier heights positioned in its close proximity. When pinch-off occurs, the potential at a“saddle point” in front of the low Schottky barrier height area determines the transport properties. At the interface between the cathode and the electron transport layer, morphology and quality of the interface can alter the barrier height. The value of barrier height can be obtained due to the changes in interfacial orientation and Fermi level pinning.

FIG. 3b shows a homogeneous Schottky barrier of one embodiment of a quantum dot light- emitting diode. Some electron spreading layers described herein are separated from cathodes described herein by homogeneous Schottky barriers like the one shown in FIG. 3b. The electron spreading layer may suppress trap sites on the cathode surface and/or may form a smooth interface with the cathode, both of which may promote the formation of a homogeneous Schottky barrier. Even discontinuous electron spreading layers may partially passivate trap sites on the cathode surface, and so may enhance the homogeneity of the Schottky barrier even if they do not cover the entirety of the cathode surface.

As described above, some electron spreading layers described herein are discontinuous. The discontinuous electron spreading layers may advantageously have a morphology that enhances the amount of light emitted from the quantum dot light-emitting diode by reducing the amount of light emitted by the light-emitting layer that is subsequently absorbed by the quantum dot light-emitting diode. FIG. 4a shows one example of a cross-sectional view of a quantum dot light-emitting diode having exclusively continuous layers. Because of Snell’s law, all of the light traveling from a material from having a higher refractive index m and toward a material having a lower refractive index m at an angle greater than the critical angle 0c will be reflected back into the higher refractive index material. This mechanism is known as total internal reflection. This means that, although some quantum dot light- emitting layers can reach an internal quantum efficiency approaching 100%, the external quantum efficiency of quantum dot light-emitting diodes including exclusively continuous layers may be limited (e.g., to at most 20%) because a portion of the light emitted by the quantum dot light-emitting layer may be emitted at an angle that cannot escape the quantum dot light-emitting diode. The light not emitted from the quantum dot light-emitting diode including only continuous layers may be lost to a variety of modes, such as waveguide modes (e.g., total internal reflection modes) and/or surface plasmon-polariton modes (e.g., modes comprising charge fluctuation of a longitudinal oscillation of a surface plasmon localized to an interface between two layers, which may be accompanied by fluctuation of transversal and longitudinal electromagnetic fields). It is believed that a large fraction of the light not emitted from the quantum dot light-emitting diode including only continuous layers (e.g., up to 80%) may be lost to waveguide modes.

FIG. 4b shows a cross-sectional view of a quantum dot light-emitting diode comprising a discontinuous electron spreading layer. The discontinuous electron spreading layer may comprise interfaces with the cathode located both parallel and non-parallel (e.g.,

perpendicular, as in FIG. 4b) to its interfaces with the quantum dot light-emitting layer.

Light entering exiting a discontinuous electron spreading layer into a cathode may pass through an interface between the discontinuous electron spreading layer and the a cathode at an angle that depends both on the position of the light wave and on the angle which the light wave entered the discontinuous electron spreading layer from the quantum dot light-emitting layer. In other words, light entering discontinuous electron spreading layers at similar angles but different positions may encounter interfaces between the electron spreading layer and the cathode at different angles. This light may thus be reflected and/or refracted differently depending on position. Some of the light that would be lost to one of the modes described above in quantum dot light-emitting diodes comprising continuous electron spreading layers may instead encounter an interface between the discontinuous electron spreading layer and the cathode that does not result in that loss. Additionally, light that is initially lost to one of the modes described above (e.g., by total internal reflection) may travel through the discontinuous electron spreading layer after reflection and then encounter another interface (e.g., between the discontinuous electron spreading layer and the quantum dot light- emitting layer, between the discontinuous electron spreading layer and the cathode) at an angle other than it would for a continuous electron spreading layer. This angle may be an angle that would promote refraction of the light into the other layer instead of further reflection that would undesirably retain the light in the layer.

Further aspects of some embodiments and their advantages can be better appreciated by the following examples.

EXAMPLES

Further aspects of some embodiments will now be described in more detail with reference to the following examples. However, the examples described herein are for purposes of explanation only and are not intended to limit the scope of the present teachings in any way.

PREPARATIVE EXAMPLE 1

Aluminum acetate for use in electron spreading layers was prepared by the procedure that follows, which was carried out under ambient conditions. 2 g of aluminum chloride was added to 10 mL of acetic acid. The solution of aluminum chloride in acetic acid was then stirred at 100 °C at 1050 rpm for 10 min under a fume hood to allow the aluminum chloride and acetic acid to fully react. The resultant aluminum acetate was centrifuged, washed with isopropyl alcohol, and then redissolved in a solvent (e.g., water and/or an alcohol, such as isopropyl alcohol, ethanol, methanol, butanol, pentanol, cetyl alcohol, and/or 2-methoxyl ethanol). Finally, the solution comprising the aluminum acetate and the solvent was filtered through a syringe filter (0.2 mhi).

COMPARATIVE EXAMPLE 1 PEDOT:PSS solutions filtered through a syringe filter (0.45 mhi) and then spin-coated onto ITO-coated glass substrates at 3000 rpm for 60 s. After which, the resultant substrates were baked at 145 °C for 15 min. The PEDOT:PSS-coated substrates were the transferred into a nitrogen-filled globe box. Next, TFB was spin-coated onto the PEDOT:PSS-coated substrates at 3000 rpm for 30 s, which were then baked at 150 °C for 30 min. Quantum dots and ZnO nanocrystals were sequentially deposited onto the TFB by spin coating at 2000 rpm for 60 s and 30 s, respectively. The quantum dots were Cd-based core/shell colloidal nanocrystals with a green color. A1 electrodes (100 nm) were deposited onto the ZnO nanocrystals using thermal evaporation through a shadow mask. Finally, the devices were encapsulated in an ultraviolet-curable resin and then covered by a glass slide.

EXAMPLE 1

PEDOT:PSS solutions were filtered through a syringe filter (0.45 mhi) and then spin-coated onto ITO-coated glass substrates at 3000 rpm for 60 s. After this step, the resultant substrates were baked at 145 °C for 15 min. The PEDOT:PSS-coated substrates were then transferred into a nitrogen-filled globe box. Next, TFB was spin-coated onto the

PEDOT:PSS coated substrates at 3000 rpm for 30 s, which were then baked at 150 °C for 30 min. Quantum dots and ZnO nanocrystals were sequentially deposited onto the TFB by spin coating at 2000 rpm for 60 s and 30 s, respectively. The quantum dots were Cd-based core/shell colloidal nanocrystals with a green color. Aluminum acetate synthesized as described in Preparative Example 1 which had been redissolved in isopropyl alcohol was then spin-coated at 2000 rpm for 30 s onto the quantum dots and ZnO nanocrystals. The resultant substrates were baked at 120 °C for 10 min under air, causing thermal

decomposition of the aluminum acetate and subsequent generation of alumina nanoparticles. A1 electrodes (100 nm) were deposited onto the alumina nanoparticles using thermal evaporation through a shadow mask. Finally, the devices were encapsulated in an ultraviolet-curable resin and then covered by a glass slide.

FIG. 5a shows the current density as a function of voltage for Example 1 and Comparative Example 1. The closed symbols show data from Example 1 and the open symbols show data from Comparative Example 1. The square, circular, and triangular symbols show data from these samples on the first day (tl.l), 6^th day (t2.6), and 14^th day (t3.14), respectively, after device fabrication. Comparative Example 1 shows significant improvement after aging for one week, displaying a positive aging effect. Although the leakage current and the total resistance are relatively high on the first day after device fabrication, these properties dramatically improve by the 6^th day after device fabrication. However, the performance of Comparative Example 1 degrades slightly under a high applied voltage at the 2^nd week after device fabrication. Example 1 has a lower leakage current than that Comparative Example 1 on the first day after device fabrication and is much more stable than Comparative Example 1. This improvement may be due to passivation of interface defects between the A1 cathode and the ZnO electron transport layer by the electron spreading layer, which may result in the formation of a homogeneous Schottky barrier. The homogeneous Schottky barrier may promote uniform electron injection. The electron spreading layer is believed to comprise continuous alumina nanoparticle thin films on the ZnO electron transport layer.

FIG. 5b shows the external quantum efficiency (EQE) as a function of applied voltage for the Example 1 and Comparative Example 1 and FIG. 5c shows EQE as a function of luminance for Example 1 and Comparative Example 1. The closed symbols show data from Example 1 and the open symbols show data from Comparative Example 1. The square, circular, and triangular symbols show data from these samples on the first day (tl.l), 6^th day (t2.6), and 14^th day (t3.14), respectively, after device fabrication. The turn-on voltage of Example 1 is much lower than that of Comparative Example 1 at both the 6^th day and the 2^nd week after device fabrication. In addition, the peak external quantum efficiency of Example 1 is higher than that of Comparative Example 1 at the 2^nd week from device fabrication. Since

Example 1 and Comparative Example 1 have similar levels of leakage current, the low turn on voltage and high peak external quantum efficiency of Example 1 could result from the elimination of waveguide and surface plasmon-polariton modes by light scattering at the alumina nanoparticle thin film on the ZnO electron transport layer. In addition, the increased total resistance Comparative Example 1 in comparison to Example 1 might cause the low external quantum efficiency and high tum-on voltage at 2^nd week from device fabrication. Although the on-set of efficiency droop (the highest value of external efficiency vs luminance) of Example 1 occurs at lower luminance in (see FIG. 5c), a wider peak of external quantum efficiency vs. luminance may provide more stable device performance than a sharper peak. COMPARATIVE EXAMPLE 2

25 nm-thick M0O3 was deposited onto ITO-coated glass substrates using thermal evaporation. The Mo0₃-coated substrates were transferred into a nitrogen-filled globe box. Next, TFB was spin-coated at 3000 rpm for 30 s onto the Mo0₃-coated substrates, which were then baked at 150 °C for 30 min. Quantum dots and ZnO nanocrystals were sequentially deposited by spin coating at 2000 rpm for 60 s and 30 s, respectively. The quantum dots were Cd-based core/shell colloidal nanocrystals with a green color. A1 electrodes (100 nm) were deposited onto the ZnO nanocrystals using thermal evaporation through a shadow mask. Finally, the devices were encapsulated in an ultraviolet-curable resin and covered by a glass slide, after which they were removed from the glove box.

EXAMPLE 2

25 nm-thick M0O3 was deposited onto ITO-coated glass substrates using thermal evaporation. The MoCE-coated substrates were transferred into a nitrogen-filled globe box. Next, TFB was spin-coated at 3000 rpm for 30 s onto the MoCE-coated substrates, which were then baked at 150 °C for 30 min. A further hole transport layer comprising nanocrystals at a concentration of 10 mg/mL in heptane was spin-coated at 2000 rpm for 30 s onto the MoCE-coated substrates, which were then baked at 150 °C for 30 min. Quantum dots and ZnO nanocrystals were sequentially deposited by spin coating at 2000 rpm for 60 s and 30 s, respectively. The quantum dots were Cd-based core/shell colloidal nanocrystals with a green color. Magnesium acetate which had been redissolved in isopropyl alcohol was then spin-coated at 2000 rpm for 30 s onto the quantum dots and ZnO nanocrystals. The resultant substrates were baked at 120 °C for 10 min under air, causing thermal

decomposition of the magnesium acetate and subsequent generation of magnesium oxide nanoparticles. A1 electrodes (100 nm) were deposited onto the magnesium oxide nanoparticles using thermal evaporation through a shadow mask. Finally, the devices were encapsulated in an ultraviolet-curable resin and covered by a glass slide, after which they were removed from the glove box.

FIG. 6a shows the current density as a function of voltage for Example 2 and Comparative Example 2. The closed symbols show data from Example 2 and the open symbols show data from Comparative Example 2. The square, circular, and triangular symbols show data from these samples on the first day (tl.l), 6^th day (t2.6), and 14^th day (t3.14), respectively, after device fabrication. Comparative Example 2 shows significant positive aging effect for one week. The leakage current and total resistance are dramatically improved on the 7^th day after device fabrication. Additionally, the built-in potential (the inflection point of current as a function of voltage when plotted on a log scale, which equals the potential across the depletion region in thermal equilibrium) of Example 2 is lower than that of Comparative Example 2. This implies that the potential barrier that opposes both the flow of holes and electrons into the quantum dot light-emitting layer is lower in Example 2 than in Comparative Example 2, and that the recombination rate of electrons and holes in the quantum dot light- emitting layer is higher in Example 2 than in Comparative Example 2. Additionally, the ballistic electron transport across the quantum dot light-emitting layer may occur in the presence of a high electric field. These hot electrons will transverse the quantum dot light- emitting layer, escape recombination inside the light-emitting layer, and contribute the electron overflow current, or be thermalized and captured inside the light-emitting layer through interaction with phonons.

FIG. 6b shows the external quantum efficiency (EQE) as a function of applied voltage for Example 2 and Comparative Example 2 and FIG. 6c shows the EQE as a function of luminance for Example 2 and Comparative Example 2. The closed symbols show data from Example 2 and the open symbols show data from Comparative Example 2. The square, circular, and triangular symbols show data from these samples on the first day (tl.l), 6^th day (t2.6), and 14^th day (t3.14), respectively, after device fabrication. Although the turn-on voltage of Example 2 is higher than that of Comparative Example 2 at the 1^st and 2^nd weeks after device fabrication, the peak external quantum efficiency of Example 2 is higher than that of Comparative Example 2 at the 1^st and 2^nd weeks after device fabrication. Both uniform electron injection due to a homogeneous Schottky barrier height and reduction of waveguide and surface plasmon-polariton modes are believed to improve the external quantum efficiency of Example 2. The magnesium oxide electron spreading layer in Example 2 causes the maximum external quantum efficiency to occur at high luminance, as shown in FIG. 6c.

FIG. 6d shows the external quantum efficiency as a function of current density for Example 2 and Comparative Example 2. The closed symbols show data from Example 2 and the open symbols show data from Comparative Example 2. The square, circular, and triangular symbols show data from these samples on the first day (tl.l), 6^th day (t2.6), and 14^th day (t3.14), respectively, after device fabrication. The curves shown in FIG. 6d can be divided into two regions. The first is the region from zero to the peak value of the external quantum efficiency, and the second is the degradation region due to the localized large current densities attributing to the overflow. Regarding region 1, it is believed that when a large number of carriers accumulate in the quantum dot light-emitting layer interface to screen the potential, the external quantum efficiency reaches its peak value due to the stronger overlapping between the electrons and holes distribution as current density increases.

Regarding region 2, it is believed that as the current increases further, the external quantum efficiency shows degradation because of the overflow. This phenomenon may be referred to as efficiency droop. Example 2 shows higher external quantum efficiency and delayed onset current in comparison to Comparative Example 2. It is believed that Example may have better current spreading and better light extraction than Comparative Example 2, and that this may cause these effects.

The description herein is merely exemplary in nature and, thus, variations that do not depart from the gist of that which is described are intended to be within the scope of the teachings. Such variations are not to be regarded as a departure from the spirit and scope of the teachings.

While several embodiments of the present invention have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the functions and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the present invention. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the teachings of the present invention is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the scope of the present invention.

All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms.

The indefinite articles“a” and“an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean“at least one.”

The phrase“and/or,” as used herein in the specification and in the claims, should be understood to mean“either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e.,“one or more” of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the“and/or” clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to“A and/or B”, when used in conjunction with open-ended language such as“comprising” can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.

As used herein in the specification and in the claims,“or” should be understood to have the same meaning as“and/or” as defined above. For example, when separating items in a list, “or” or“and/or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as“only one of’ or “exactly one of,” or, when used in the claims,“consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term“or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e.“one or the other but not both”) when preceded by terms of exclusivity, such as“either,”“one of,”“only one of,” or “exactly one of.”“Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase“at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase“at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example,“at least one of A and B” (or, equivalently,“at least one of A or B,” or, equivalently“at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

It should also be understood that, unless clearly indicated to the contrary, in any methods claimed herein that include more than one step or act, the order of the steps or acts of the method is not necessarily limited to the order in which the steps or acts of the method are recited.

In the claims, as well as in the specification above, all transitional phrases such as “comprising,”“including,”“carrying,”“having,”“containing,”“involving,”“holding,” “composed of,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases“consisting of’ and“consisting essentially of’ shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.

Claims

CLAIMS What is claimed is:

1. A quantum dot light-emitting diode comprising: a first electrode; a quantum dot light-emitting layer disposed on the first electrode; an electron transport layer disposed on the quantum dot light-emitting layer; an electron spreading layer disposed on the electron transport layer; and a second electrode disposed on the electron spreading layer.

2. The quantum dot light-emitting diode of claim 1, wherein the quantum dot light- emitting diode further comprises a hole transport layer positioned between the first electrode and the quantum dot light-emitting layer.

3. The quantum dot light-emitting diode of any preceding claim, wherein the electron spreading layer comprises an oxide.

4. The quantum dot light-emitting diode of claim 3, wherein the oxide comprises one or more of AI2O3, MgO, HfC , GaiCL, GdiCL, La203, S1O2, Zr02, Y2O3, Ta203, T1O2, and BaO.

5. The quantum dot light-emitting diode of any preceding claim, wherein the electron spreading layer comprises a nitride.

6. The quantum dot light-emitting diode of claim 5, wherein the nitride comprises S13N4.

7. The quantum dot light-emitting diode of any preceding claim, wherein the electron spreading layer comprises a carbide.

8. The quantum dot light-emitting diode of any claim 7, wherein the carbide comprises SiC.

9. The quantum dot light-emitting diode of any preceding claim, wherein the electron spreading layer comprises a sulfide.

10. The quantum dot light-emitting diode of claim 9, wherein the sulfide comprises ZnS.

11. The quantum dot light-emitting diode of any preceding claim, wherein the electron spreading layer comprises a halide salt.

12. The quantum dot light-emitting diode of claim 11, wherein the halide salt comprises LiF, LiCl, CuCl, and/or MgF₂.

13. The quantum dot light-emitting diode of any preceding claim, wherein the electron spreading layer comprises an organic material.

14. The quantum dot light-emitting diode of claim 13, wherein the organic material comprises one or more of PVK, PMMA, and polystyrene.

15. The quantum dot light-emitting diode of any preceding claim, wherein the electron spreading layer is continuous.

16. The quantum dot light-emitting diode of claim 15, wherein the electron spreading layer has a thickness of less than 10 nm.

17. The quantum dot light-emitting diode of any one of claims 1-14, wherein the electron spreading layer is discontinuous.

18. The quantum dot light-emitting diode of claim 17, wherein the electron spreading layer comprises pores.

19. The quantum dot light-emitting diode of any one of claims 17-18, wherein the electron spreading layer has a thickness of less than 10 nm.

20. A method of manufacturing a quantum dot light-emitting diode, comprising: assembling an electron spreading layer with an electron transport layer, a first electrode, a second electrode, and a quantum dot light-emitting layer.

21. The method of claim 20, wherein assembling the electron spreading layer with the quantum dot light-emitting layer comprises depositing the electron spreading layer onto the quantum dot light emitting layer

22. The method of claim 21, further comprising dispersing a precursor in a dispersion solvent.

23. The method of claim 22, wherein the precursor comprises a nitrate salt and/or an acetate salt.

24. The method of claim 23, wherein the acetate salt comprises one or more of an A1 cation, a Mg cation, a Hf cation, a Ga cation, a Gd cation, a La cation, a Si cation, a Zr cation, an Y cation, a Ta cation, a Ti cation, and a Ba cation.

25. The method of any one of claims 23-24, wherein the nitrate salt comprises one or more of an A1 cation, a Mg cation, a Hf cation, a Ga cation, a Gd cation, a La cation, a Si cation, a Zr cation, an Y cation, a Ta cation, a Ti cation, and a Ba cation.

26. The method of any one of claims 22-25, wherein the dispersion comprises water and/or an alcohol.

27. The method of claim 26, wherein the alcohol is isopropyl alcohol, ethanol, methanol, butanol, pentanol, cetyl alcohol, and/or 2-methoxyl ethanol.

28. The method of any one of claims 22-27, wherein depositing the electron spreading layer comprises depositing the dispersed precursor in the dispersion solvent on the quantum dot light-emitting layer.

29. The method of any one of claims 22-28, wherein the depositing the precursor comprises a solution coating process.

30. The method of claim 29, wherein the solution coating process comprises sol-gel coating, spin coating, printing, casting, stamping, dip coating, roll-to-roll coating, and/or spraying.

31. The method of any one of claims 22-30, further comprising decomposing the precursor to form the electron spreading layer.

32. The method of claim 31, wherein the decomposing the precursor comprises one or more of thermal decomposition, ozone treatment, and oxygen plasma treatment.

33. The method of claim 32, wherein thermal decomposition comprises heating the precursor to a temperature of greater than or equal to 50 °C and less than or equal to 200 °C for a time period of greater than or equal to 1 min and less than or equal to 100 hr in air and/or oxygen.

34. The method of any one of claims 20-33, wherein the quantum dot light-emitting diode further comprises a hole transport layer positioned between the first electrode and the quantum dot light-emitting layer.

35. The method of any one of claims 20-34, wherein the electron spreading layer comprises an oxide.

36. The method of claim 35, wherein the oxide comprises one or more of AI2O3, MgO, HfC , Ga203, Gd203, La203, S1O2, Zr02, Y2O3, Ta203, T1O2, and BaO.

37. The method of any one of claims 20-36, wherein the electron spreading layer comprises a nitride.

38. The method of claim 37, wherein the nitride comprises S13N4.

39. The method of any one of claims 20-38, wherein the electron spreading layer comprises a carbide.

40. The method of claim 39, wherein the carbide comprises SiC.

41. The method of any one of claims 20-40, wherein the electron spreading layer comprises a sulfide.

42. The method of claim 41, wherein the sulfide comprises ZnS.

43. The method of any one of claims 20-42, wherein the electron spreading layer comprises a halide salt.

44. The method of claim 43, wherein the halide salt comprises LiF, LiCl, CuCl, and/or

MgF2.

45. The method of any one of claims 20-44, wherein the electron spreading layer comprises an organic material.

46. The method of claim 45, wherein the organic material comprises one or more of PVK, PMMA, and polystyrene.

47. The method of any one of claims 20-46, wherein the electron spreading layer is continuous.

48. The method of claim 47, wherein the electron spreading layer has a thickness of less than 10 nm.

49. The method of any one of claims 20-46, wherein the electron spreading layer is discontinuous.

50 The method of claim 49, wherein the electron spreading layer comprises pores.

51. The method of any one of claims 49-50, wherein the electron spreading layer has a thickness of less than 10 nm.

52. A quantum dot light-emitting diode formed by the method of any one of claims 20- 51.