WO2020142482A1

WO2020142482A1 - Quantum dot light-emitting diodes comprising hole transport layers

Info

Publication number: WO2020142482A1
Application number: PCT/US2019/069051
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: TW202044608A

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 hole injection layer disposed on the first electrode, a hole transport layer comprising ZnS disposed on the hole injection layer, a quantum dot light-emitting layer disposed on the hole transport layer, and a second electrode disposed on the quantum dot light-emitting layer. A method may comprise assembling a hole transport layer comprising ZnS with a hole injection layer, a quantum dot light-emitting layer, a first electrode, and a second electrode.

Description

Quantum Dot Light-Emitting Diodes Comprising Hole Transport Layers

FIELD

The present disclosure relates to hole transport layers, and, more particularly, hole transport layers comprising zinc sulfide.

BACKGROUND

The statements in this section merely provide background information related to the present disclosure and do not necessarily describe prior art.

Quantum dot light-emitting diodes are capable of producing light upon application of a voltage thereto. Typically, the quantum dots emit the light, and are sandwiched between an electron transport layer and a hole transport layer. An applied voltage may provide cause electrons and holes to flow into the quantum dot layer, where they may be captured and recombined, generating photons. Current hole transport layers for use in these devices suffer drawbacks, such as conducting holes to a lower degree than commonly-used electron transporting layers. This may undesirably cause charge to accumulate on the quantum dots, resulting in nonradiative and Auger recombination process. Accordingly, improved hole transport layers are needed.

SUMMARY

Hole transport 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 hole injection layer disposed on the first electrode, a hole transport layer comprising ZnS disposed on the hole injection layer, a quantum dot light-emitting layer disposed on the hole transport layer, and a second electrode disposed on the quantum dot light-emitting layer.

In some embodiments, a method of manufacturing a quantum dot light-emitting diode is provided. The method comprises assembling a hole transport layer comprising ZnS with a hole injection layer, a quantum dot light-emitting layer, a first electrode, and a second electrode.

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.

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.

FIG. 1 is a schematic cross-sectional view of a quantum dot light-emitting diode comprising a hole transport layer according to one embodiment;

FIG. 2 is a graph showing the current density as a function of applied voltage for the quantum dot light-emitting diode described in Example 1 ;

FIG. 3 is a graph depicting the external quantum efficiency as a function of voltage for the quantum dot light-emitting diode described in Example 1 ;

FIG. 4 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. 5 is a graph showing the external quantum efficiency as a function of applied voltage for the quantum dot light-emitting diode described in Example 2;

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

FIG. 7 is a graph depicting the external quantum efficiency as a function of voltage for the quantum dot light-emitting diode described in Example 3;

FIG. 8 is a graph showing the external quantum efficiency as a function of luminance for the quantum dot light-emitting diode described in Example 3; and

FIG. 9 is a graph showing the external quantum efficiency as a function of luminance for the quantum dot light-emitting diode described in Example 4.

DETAILED DESCRIPTION

The present disclosure generally relates to hole transport layers for quantum dot light- emitting diodes and associated articles and methods. The hole transport layers may be configured such that holes supplied on one side thereof, such as a side opposite a quantum dot layer light-emitting layer, are transferred therethrough, such as to the quantum dot light- emitting layer and/or at least a portion of the quantum dots in a quantum dot light-emitting layer. When some hole transport layers described herein are positioned in quantum dot light-emitting diodes, they advantageously increase the performance of the quantum dot light- emitting diodes. For instance, some hole transport layers described herein may provide enhanced charge transport in comparison to other hole transport layers, which may beneficially cause a reduced imbalance in charge transport to a quantum dot layer in comparison to other quantum dot light-emitting diodes and/or may cause balanced charge transport to a quantum dot layer. Balanced charge transport may desirably reduce quantum dot charging and reduce nonradiative Auger recombination processes, which may enhance the electroluminescent efficiency of the quantum dot light-emitting diode.

When employed in quantum dot light-emitting diodes, the hole transport layers described herein may promote balanced charge transport because they may have a highest occupied molecular orbital (HOMO) with an advantageous energy. The energy of the HOMO of the hole transport layer may be close to and/or above that of the valence band of the quantum dots in the light emitting layer, promoting hole transfer from the hole transport layer to the quantum dots (e.g., it may be greater than or equal to 4.7 eV). In some embodiments, the hole transport layer may have a relatively high hole mobility (e.g., in excess of 0.001 cm²/(V*s)), promoting hole transport therethrough. The hole transport layer may comprise a material, such as ZnS, that inherently has a relatively high hole mobility and/or a HOMO with an energy that is close to that of the valence band of the quantum dots in the quantum dot light-emitting layer. In some embodiments, the hole transport layer is doped with a type of dopant and/or with an amount of dopant that further enhances the hole mobility of the hole transport layer and/or that cause the energy of the HOMO of the hole transport layer to be close to that of the valence band of the quantum dot light-emitting layer.

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 is a schematic view of a quantum dot light-emitting diode according to one

embodiment. In FIG. 1, the quantum dot light-emitting diode comprises a substrate 10, an anode 20 disposed on the substrate, a hole injection layer 30 disposed on the anode, a hole transport layer 40 disposed on the hole injection layer, a quantum dot light-emitting layer 50 disposed on the hole transport layer, an electron transport layer 60 disposed on the quantum dot light-emitting layer, and a cathode 70 disposed on the electron transport 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 20 may inject holes into the hole injection layer 30. The holes may then be transported through the hole transport layer 40. Application of a voltage may also cause the cathode 70 to inject electrons into the electron transport layer 60, through which the electrons may be transported. The injected holes and injected electrons may combine in the quantum dot light-emitting layer 50 (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, and a cathode may be deposited on the electron transport layer), in the reverse order (i.e., an electron transport layer may be deposited on a cathode, 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, 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, Si02, Zr02, Y2O3, Ta203, Ti02, 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 MgF2). Further non-limiting examples of suitable ceramics include copper(I) thiocyanate (CuSCN), copper gallium oxide (CuGa0₂), and copper aluminum oxide (CUAIO2). 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 variety of ceramics. In some embodiments, the hole transport layer comprises an optionally-doped semiconductor. For instance, the hole transport layer may comprise an optionally-doped Group II- VI compound, such as doped or undoped ZnS. In some embodiments, the ZnS is doped with one or more of Mg, P, As, or Sb. Non-limiting examples of suitable types of doped ZnS include Cu-doped ZnS (Cu_xZni- _xS), Ag-doped ZnS (Ag_xZni-_xS), and Au-doped ZnS (Au_xZni-_xS). Non-limiting examples of suitable types of doped ZnS include Cu-doped ZnS (Cu_xZni-_xS), Mg-doped ZnS (Mg_xZni-_xS), Ag-doped ZnS (Ag_xZni-_xS), and Au-doped ZnS (Au_xZni-_xS). The hole transport layer may include a polymer and/or an organic molecule (e.g., in a relatively low amount, such as below 20 wt%, below 10 wt%, below 5 wt%, below 2 wt%, or below 1 wt%), or may be polymer- free and/or organic molecule-free. Non-limiting examples of suitable polymers that may be included in the hole transport layer include poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N- (4-sec-butylphenyl)) diphenylamine)] (TFB), poly(9-vinylcarbazole) (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 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). 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.

When the hole transport layer comprises a dopant, it may comprise the dopant in a variety of suitable amounts. In some embodiments, a hole transport layer comprises doped ZnS, and the molar ratio of the dopant to Zn is greater than or equal to 0.01, greater than or equal to 0.0125, greater than or equal to 0.015, greater than or equal to 0.02, greater than or equal to 0.025, greater than or equal to 0.03, greater than or equal to 0.04, greater than or equal to 0.05, greater than or equal to 0.075, greater than or equal to 0.1, greater than or equal to 0.125, greater than or equal to 0.15, greater than or equal to 0.2, greater than or equal to 0.25, greater than or equal to 0.3, or greater than or equal to 0.4. In some embodiments, a hole transport layer comprises doped ZnS, and the molar ratio of the dopant to Zn is less than or equal to 0.5, less than or equal to 0.4, less than or equal to 0.3, less than or equal to 0.25, less than or equal to 0.2, less than or equal to 0.15, less than or equal to 0.125, less than or equal to 0.1, less than or equal to 0.075, less than or equal to 0.05, less than or equal to 0.04, less than or equal to 0.03, less than or equal to 0.025, less than or equal to 0.02, less than or equal to 0.015, or less than or equal to 0.0125. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 0.01 and less than or equal to 0.5). Other ranges are also possible. When a quantum dot light-emitting diode comprises two or more hole transport layers, each hole transport layer may independently comprise an amount of dopants in one or more of the ranges 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 an 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 an 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 an average diameter in one or more of the ranges described above.

In some embodiments, it is desirable for the hole transport layer to have a band gap larger than that of the energy of light emitted from the quantum dot light-emitting layer. Without wishing to be bound by any particular theory, it is believed that this may prevent absorption of the light emitted by the quantum dot light-emitting diode by the hole transport layer.

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 from 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, and/or HgTe. 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. Non limiting examples of suitable Group III-V compound semiconductor nanocrystal quantum dots include those comprising GaP, GaAs, InP, and/or InAs. In some embodiments, a quantum dot light-emitting layer comprises a Group IV- VI compound semiconductor quantum dot, such as a 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, and/or PbTe. 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 an 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 an 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, 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 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 an 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 an 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. 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, or 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 or more 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 by reference herein in its entirety for all purposes.

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/or 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 200 °C in nitrogen, argon, helium, 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.

Thermal annealing as described herein may be carried out in the presence of nitrogen, argon, helium, air and/or oxygen.

In some embodiments, a quantum-dot light emitting diode described herein has one or more advantageous properties. For instance, as described above, some quantum dot light-emitting diodes exhibit reduced quantum dot charging and/or reduced nonradiative Auger

recombination in comparison to other quantum dot light-emitting diodes. In other words, they may have a higher radiative efficiency than other quantum dot light-emitting diodes.

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

Cu-doped ZnS nanocrystals for use in a hole transport layer were prepared by the procedure described below, throughout which standard Schlenk techniques were employed. 55 mg of zinc acetate and 0.189 mL of oleic acid were dissolved with 0.189 mL of oleic acid in 20 mL of trioctylamine (TOA). The mole ratio of Cu to Zn was 0.25 in the resultant solution.

This solution was then heated to 60 °C and degassed. Afterward, 0.486 mL of 1- hexadecanethiol was added and degassed. After completely degassing, the flask was heated to 240 °C under vigorous stirring. During ramping up, when the color of solution changed, the temperature was rapidly lowered to room temperature by means of a flow of compressed air. The nanocrystals were isolated by addition of excess acetone, centrifuged, and washed several times with acetone. The as-prepared nanocrystals were redispersed in heptane.

The solution containing the nanocrystals was filtered through a syringe filter (0.2 mhi).

COMPARATIVE 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 which, 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. 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 which, 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. A fluid comprising the Cu-doped ZnS nanocrystals described in Preparative Example 1 at a concentration of 5 mg/mL in heptane was spin-coated at 4000 rpm for 30 s onto the TFB, and then the resultant structure was baked at 150 °C for 30 min. Quantum dots and ZnO nanocrystals were sequentially deposited onto the CuZnS nanocrystals 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. The device structure is depicted in FIG 1.

FIG. 2 shows the current-voltage characteristics of the quantum-dot LEDs described in Comparative Example 1 and Example 1 at 52 days after device fabrication. The solid line and dashed line show data from Example 1 and Comparative Example 1, respectively.

Under a high applied voltage, the two devices have similar total resistances. However, the leakage current of device with Cu_xZni-_xS is lower than that of device without Cu_xZni-_xS.

This means that the quantum dot light-emitting diodes with Cu_xZni-_xS have a low defect density. FIG. 3 shows the external quantum efficiency (EQE) as a function of applied voltage. The turn-on voltage of the quantum dot light-emitting diodes with Cu_xZni-_xS is lower than that of the quantum dot light-emitting diodes without Cu_xZni-_xS. In addition, the peak external quantum efficiency of the quantum dot light-emitting diodes with Cu_xZni-_xS is higher than that of the quantum dot light-emitting diodes without Cu_xZni-_xS, as shown in FIG. 4. Since Cu_xZni-_xS has larger work function (-6.25 eV) than TFB (-5.3 eV), the on set of efficiency droop (the highest value of external efficiency vs luminance) for the quantum dot light-emitting diodes with Cu_xZni-_xS is higher.

PREPARATIVE EXAMPLE 2

Cu-doped ZnS nanocrystals for use in a hole transport layer were prepared by the procedure described below, throughout which standard Schlenk techniques were employed. 55 mg of zinc acetate and 21.8 mg of copper acetate were dissolved with 0.189 mL of oleic acid in 20 mL of TOA. The mole ratio of Cu to Zn was 0.40 in the resultant solution. This solution was then heated to 120 °C and degassed. Afterward, 0.278 mL of 1-octanethiol was added, and the solution was again degassed. After completely degassing, the flask was heated to 220 °C under vigorous stirring. During ramping up, when the color of solution changed, the temperature was rapidly lowered to room temperature by means of a flow of compressed air. For additional ligand exchange, the solution was reheated to a temperature of 120 °C, after which 1 mL of additional 1-octanethiol was injected. After 60 min, the solution was allowed to cool to room temperature by exchanging heat with an ambient room temperature environment. The nanocrystals were then isolated by addition of excess acetone, centrifuged, and washed several times with acetone. The as-prepared nanocrystals were redispersed in heptane. The solution containing the nanocrystals is filtered through a syringe filter (0.2 mhi).

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 were deposited onto ITO-coated glass substrates using thermal evaporation. The Mo0₃-coated substrates were transferred into a nitrogen-filled globe box. TFB was then spin-coated onto the M0O3 at 3000 rpm for 30 s, after which the coated substrates were baked at 150 °C for 30 min. A fluid comprising the Cu-doped ZnS nanocrystals described in Preparative Example 2 at a concentration of 10 mg/mL in heptane was spin-coated at 2000 rpm for 30 s onto the TFB, and then the resultant structure was baked at 150 °C for 30 min. Quantum dots and ZnO nanocrystals were sequentially deposited onto the Cu-doped ZnS nanocrystals 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 using thermal evaporation through a shadow mask. Finally, the devices were encapsulated in an ultraviolet-curable resin and then covered by a glass slide, after which they were removed from the glove box.

FIG. 5 shows the external quantum efficiency as a function of applied voltage for the quantum-dot LEDs described in Comparative Example 2 and Example 2 on the first day after device fabrication. The solid line and dashed line show data from Example 2 and

Comparative Example 2, respectively. The highest external quantum efficiency of the quantum dot light-emitting diodes with Cu_xZni-_xS is higher than that of the quantum dot light-emitting diodes without Cu_xZni-_xS. In addition, the peak external quantum efficiency of the quantum dot light-emitting diodes with Cu_xZni-_xS is the same as that of the quantum dot light-emitting diodes without Cu_xZni-_xS, as shown in FIG. 6. Additionally, the highest value of external quantum efficiency of the quantum dot light-emitting diodes with Cu_xZni-_xS is higher than that of the quantum dot light-emitting diodes lacking Cu_xZni-_xS at the same luminance. Without wishing to be bound by any particular theory, it is believed that charge carriers are easier to transport through 1-octanethiol than through other, longer ligands because shorter ligands experience less bond-bending and less electron trapping in states localized on the long ligands.

PREPARATIVE EXAMPLE 3

Cu-doped ZnS nanocrystals for use in a hole transport layer were prepared by the procedure described below, throughout which standard Schlenk techniques were employed. 55 mg of zinc acetate and 15.7 mg of copper acetylacetone were dissolved with 0.189 mL of oleic acid in 20 mL of trioctylamine (TOA). The mole ratio of Cu to Zn was 0.10 in the resultant solution. This solution was then heated to 80 °C and degassed. Afterward, 0.278 mL of 1- dodecanethiol was added and degassed. After completely degassing, the flask was heated to 300 °C under vigorous stirring. After 150 min, the temperature was rapidly lowered to room temperature by means of a flow of compressed air. The nanocrystals were isolated by addition of excess acetone, centrifuged, and washed several times with acetone. The as- prepared nanocrystals were redispersed in chloroform. The solution containing the nanocrystals is filtered through a syringe filter (0.2 mhi).

EXAMPLE 3

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 which, 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, a fluid comprising the Cu-doped ZnS nanocrystals described in Preparative Example 3 at a concentration of 8 mg/mL in chloroform and comprising TFB was spin-coated onto the PEDOT:PSS-coated substrates at 2000 rpm for 30 s, which were then baked at 150 °C for 30 min. Quantum dots and doped ZnO nanocrystals were sequentially deposited onto the previously-deposited layer 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.

FIG. 7 shows the external quantum efficiency as a function of applied voltage for the quantum dot LED described in Example 3 at 7 days after device fabrication. FIG. 8 shows that the highest value of EQE occurs at 6392 Cd/m² of luminance.

PREPARATIVE EXAMPLE 4

Ag-doped ZnS nanocrystals for use in a hole transport layer were prepared by the procedure described below, throughout which standard Schlenk techniques were employed. 55 mg of zinc acetate and 20.4 mg of silver nitrate were dissolved with 0.189 mL of oleic acid in 20 mL of trioctylamine (TOA). The mole ratio of Ag to Zn was 0.40 in the resultant solution. This solution was heated to 120 °C and degassed. Afterward, 0.278 mL of 1-octanethiol was added and degassed. After completely degassing, the flask was heated to 300 °C under vigorous stirring. When the temperature reached 300 °C, it was subsequently rapidly lowered to room temperature by means of a flow of compressed air. For additional ligand exchange, the solution was reheated to a temperature of 120 °C, after which 1 mL of additional 1-octanethiol was injected. After 60 min, the solution was allowed to cool to room temperature by exchanging heat with an ambient room temperature environment. The nanocrystals were isolated by addition of excess acetone, centrifuged, and washed several times with acetone. The as-prepared nanocrystals were redispersed in heptane. The solution containing the nanocrystals is filtered through a syringe filter (0.2 mhi).

EXAMPLE 4

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 which, 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. A fluid comprising the Ag-doped ZnS nanocrystals described in Preparative Example 4 at a concentration of 2 mg/mL in heptane was spin-coated at 4000 rpm for 30 s onto the TFB, and then the resultant structure was baked at 150 °C for 30 min. Quantum dots and ZnO nanocrystals were sequentially deposited onto the CuZnS nanocrystals 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, after which they were removed from the glove box. The device structure is depicted in FIG 1.

FIG. 9 shows the EQE as a function of luminance in Example 4 one day after device fabrication.

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 hole injection layer disposed on the first electrode; a hole transport layer comprising ZnS disposed on the hole injection layer; a quantum dot light-emitting layer disposed on the hole transport layer; and a second electrode disposed on the quantum dot light-emitting layer.

2. The quantum dot light-emitting diode of claim 1, wherein the quantum dot light- emitting diode comprises an electron transport layer positioned between the quantum dot light-emitting layer and the second electrode.

3. The quantum dot light-emitting diode of any preceding claim, wherein the ZnS is doped.

4. The quantum dot light-emitting diode of claim 3, wherein the doped ZnS is doped with Cu.

5. The quantum dot light-emitting diode of claim 4, wherein a molar ratio of Cu to Zn in the hole transport layer is greater than or equal to 0.01 and less than or equal to 0.5.

6. The quantum dot light-emitting diode of any one of claims 3-5, wherein the doped ZnS is doped with Ag.

7. The quantum dot light-emitting diode of claim 6, wherein a molar ratio of Ag to Zn is greater than or equal to 0.01 and less than or equal to 0.5.

8. The quantum dot light-emitting diode of any one of claims 3-7, wherein the doped ZnS is doped with Au.

9. The quantum dot light-emitting diode of claim 8, wherein a molar ratio of Au to Zn in the hole transport layer is greater than or equal to 0.01 and less than or equal to 0.5.

10. The quantum dot light-emitting diode of any preceding claim, wherein the hole transport layer comprises nanoparticles.

11. The quantum dot light-emitting diode of claim 10, wherein the nanoparticles are crystalline.

12. The quantum dot light-emitting diode of any one of claims 10-11, wherein the nanoparticles have an average diameter of less than or equal to 200 nm.

13. The quantum dot light-emitting diode of any one of claims 10-12, wherein the nanocrystals were prepared by solution synthesis.

14. A method of manufacturing a quantum dot light-emitting diode, comprising: assembling a hole transport layer comprising ZnS with a hole injection layer, a quantum dot light-emitting layer, a first electrode, and a second electrode.

15. The method of claim 14, wherein assembling the hole transport layer with the hole injection layer comprises depositing the hole transport layer onto the hole injection layer.

16. The method of claim 15, wherein depositing the hole transport layer onto the hole injection layer comprises spin coating the hole transport layer onto the hole injection layer.

17. The method of any one of claims 14-16, wherein assembling the hole transport layer with the quantum dot light-emitting layer comprises depositing the quantum dot light- emitting layer onto the hole transport layer.

18. The method of claim 17, wherein depositing the quantum dot light-emitting layer onto the hole transport layer comprises spin coating the quantum dot light-emitting layer onto the hole transport layer.

19. The method of any one of claims 14-18, wherein assembling the quantum dot light- emitting layer with the second electrode comprises depositing the second electrode onto the quantum dot light-emitting layer.

20. The method of claim 19, wherein depositing the second electrode onto the quantum dot light-emitting layer comprises thermally evaporating the second electrode onto the quantum dot light-emitting layer.

21. The method of any one of claims 14-20, wherein assembling the first electrode with the hole injection layer comprises depositing the hole injection layer onto the first electrode.

22. The method of claim 21, wherein depositing the hole injection layer onto the first electrode comprises spin coating the hole injection layer onto the first electrode.

23. The method of any one of claims 14-22, further comprising assembling an electron transport layer with the hole transport layer, the hole injection layer, the quantum dot light- emitting layer, the first electrode, and the second electrode.

24. The method of claim 23, wherein assembling the electron transport layer comprises depositing the electron transport layer onto the quantum dot light-emitting layer.

25. The method of claim 24, wherein depositing the electron transport layer onto the quantum dot light-emitting layer comprises spin coating the electron transport layer onto the quantum dot light-emitting layer.

26. The method of any one of claims 14-25, wherein the ZnS is doped.

27. The method of claim 26, wherein the doped ZnS is doped with Cu.

28. The method of claim 27, wherein a molar ratio of Cu to Zn in the hole transport layer is greater than or equal to 0.01 and less than or equal to 0.5.

29. The method of any one of claims 26-28, wherein the doped ZnS is doped with Ag.

30. The method of claim 29, wherein a molar ratio of Ag to Zn is greater than or equal to

0.01 and less than or equal to 0.5.

31. The method of any one of claims 26-30, wherein the doped ZnS is doped with Au.

32. The method of claim 31, wherein a molar ratio of Au to Zn in the hole transport layer is greater than or equal to 0.01 and less than or equal to 0.5.

33. The method of any one of claims 14-32, wherein the hole transport layer comprises nanoparticles.

34. The method of claim 33, wherein the nanoparticles are crystalline.

35. The method of any one of claims 33-34, wherein the nanoparticles have an average diameter of less than or equal to 200 nm.

36. A quantum dot light-emitting diode made by the method of any one of claims 14-35.