WO2020142480A1

WO2020142480A1 - Quantum dot light-emitting diodes comprising doped zno electron transport layer

Info

Publication number: WO2020142480A1
Application number: PCT/US2019/069049
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: TW202036928A

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, and second electrode disposed on the electron transport layer. The electron transport layer may comprise nanoparticles comprising ZnO doped with one or more dopants and/or capped with one or more ligands. The quantum dot light-emitting layer may comprise two or more electron transport layers, each of which may independently comprise ZnO doped with one or more dopants and/or capped with one or more ligands. The nanoparticles may have a number average diameter of less than or equal to 5 nm.

Description

Quantum Dot Light-Emitting Diodes Comprising Doped ZnO Electron Transport Layer

FIELD

The present disclosure relates to quantum dot light-emitting diodes, and, more particularly, to doped electron transport layers therefor.

BACKGROUND

The statements in this section merely provide background information related to the present disclose and may not constitute prior art.

Quantum dot light-emitting diodes are capable of producing light upon application of a voltage thereto. Existing quantum dot light-emitting diodes typically emit light from quantum dots in a quantum dot light-emitting layer which is positioned between two electrodes. An applied voltage may provide cause electrons and holes to flow into the quantum dot layer from these electrodes, where they may be captured and recombined, generating photons. Some quantum dot light-emitting diodes further include electron transport layers to aid transport of electrons from the cathode to the quantum dot light- emitting layer. However, existing electron transport layers suffer from drawbacks, such as work functions that are undesirably low, reducing the efficiency of electron transport.

Accordingly, improved electron transport layers are needed.

SUMMARY

Doped ZnO electron 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 quantum dot light-emitting layer disposed on the first electrode, an electron transport layer disposed on the quantum dot light-emitting layer, and a second electrode disposed on the electron transport layer. The electron transport layer comprises nanoparticles comprising ZnO doped with one or more dopants, and the nanoparticles have a number average diameter of less than or equal to 5 nm.

In some embodiments, a method of manufacturing a quantum dot light-emitting diode is provided. The method comprises assembling an electron transport layer with a first electrode, a second electrode, and a quantum dot light-emitting layer. The electron transport layer comprises nanoparticles comprising ZnO doped with one or more dopants, and the nanoparticles have a number average diameter of less than or equal to 5 nm.

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 accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. 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 graph showing the current density as a function of voltage for the quantum dot light-emitting diodes described in Example 1 and Comparative Example 1 ;

FIG. 3 is a graph showing the external quantum efficiency as a function of luminance for the quantum dot light-emitting diodes described in Example 1 and Comparative Example 1 ;

FIG. 4 is a graph showing the current efficiency as a function of luminance for the quantum dot light-emitting diodes described in Example 1 and Comparative Example 1 ;

FIG. 5 is a graph showing the T50 lifetime as a function of luminance for the quantum dot light-emitting diodes described in Example 1 and Comparative Example 1 ;

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

FIG. 7 is a graph showing the external quantum efficiency as a function of luminance for the quantum dot light-emitting diodes described in Example 2 and Comparative Example 2;

FIG. 8 is a graph showing the current efficiency as a function of luminance for the quantum dot light-emitting diodes described in Example 2 and Comparative Example 2;

FIG. 9 is a graph showing the current density as a function of voltage for the quantum dot light-emitting diodes described in Example 3 and Comparative Example 3;

FIG. 10 is a graph showing the external quantum efficiency as a function of current density for the quantum dot light-emitting diodes described in Example 3 and Comparative Example

3;

FIG. 11 is a graph showing the current density as a function of voltage for the quantum dot light-emitting diodes described in Example 4; and

FIG. 12 is a graph showing the external quantum efficiency as a function of current density for the quantum dot light-emitting diode described in Example 4.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

The present disclosure generally relates to doped ZnO electron transport layers for quantum dot light-emitting diodes, and associated articles and methods. An electron transport layer may be positioned between a cathode, from which electrons are injected into the quantum dot light-emitting diode, and a quantum dot light-emitting layer, in which electrons are recombined with holes transport from hole transport layer to generate the photons. Some electron transport layers described herein may have advantageously high values of electronic conductivity. For instance, some electron transport layers may comprise one or more dopants that enhance the electronic conductivity of the electron transport layer as a whole. Electron transport layers having high values of electronic conductivity may beneficially promote transport of electrons through the electron transport layer in large numbers.

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 show 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 110, an anode 120 disposed on the substrate, a hole injection layer 130 disposed on the anode, a hole transport layer 140 disposed on the hole injection layer, a quantum dot light-emitting layer 150 disposed on the hole transport layer, an electron transport layer 160 disposed on the quantum dot light-emitting layer, and a cathode 170 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 may be positioned between two layers shown in FIG. 1. By way of example, in some embodiments a quantum dot light-emitting diode 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, a 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 120 may inject holes into the hole injection layer 130. The holes may then be transported through the hole transport layer 140. Application of a voltage may also cause the cathode 170 to inject and be transported through the electron transport layer 160. The injected holes and injected electrons may combine in the quantum dot light-emitting layer 150 (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, 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 MgF2). 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), poly(9-vinylcarbazole) (PVK), poly(N,N’-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine) (poly-TPD), poly[9-sec -butyl-2, 7- difluoro-9H -carbazole] (PVF), poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO), poly[(9,9-bis(3'- (N,N -dimethylamino)propyl)-2,7- fluorene)-alt -2,7-(9,9-dioctylfluorene)] (PFN-DOF), poly[(9,9-bis(3'-((N,N -dimethyl)-N -ethylammonium)- propyl)-2,7-fluorene)-alt -2,7-(9,9- dioctylfluorene)] (PFNBr), 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), NPB (N,N'-Di(l-naphthyl)-N,N'-diphenyl-(l,l'-biphenyl)-4,4'-diamine), 4,4',4"-Tris(carbazol-9-yl)triphenylamine (TCTA), 4'-Bis(carbazol-9-yl)biphenyl (CBP), 3,3'- Di(9H-carbazol-9-yl) biphenyl (mCBP), and l,3-Bis(carbazol-9-yl)benzene (mCP). Non limiting examples of suitable ceramics include copper(I) iodide (Cul), 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 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 (e.g., capping 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, 1,10-decanedithiol, a silane (e.g., 3-aminopropyltriethoxysilane, triethoxysilylbutyraldehyde, 3- isocyanatopropyltriethoxysilane, 3-mercaptopropylthrimethoxysilane, 11- cyanoundecyltrimethoxysilane, 3-acryloxypropyltrimethoxysilane, and 2- [ methoxy(polyethyleneoxy)propyl]trimethoxysilane), triethoxy silylundecanal, and n- (trimethoxysilylpropyl)ethylene diamine triacetic acid. 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 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, 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 alloy 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, and/or PbTe. Some quantum dots may be Cd-free and/or substantially Cd-free (e.g., some quantum dots may comprise Cd- free InP).

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.

As described above, the quantum dot light-emitting diodes described herein may comprise a doped ZnO electron transport layer. The electron transport layer may comprise a variety of suitable dopants. In some embodiments, an electron transport layer comprises an n-type dopant. Without wishing to be bound by any particular theory, it is believed that such dopants may advantageously decrease the Fermi energy and the work function of the electron transport layer. It is believed that this reduction may facilitate electron transport

therethrough and may cause the formation of an ohmic contact between the electron transport layer and the quantum dot light-emitting layer, which may, by providing more electrons to the quantum dot light-emitting layer, increase the charge recombination efficiency of the quantum of light-emitting diode. Non-limiting examples of suitable dopants include Group 1 dopants (e.g., Li, Na, K, Rb, and/or Cs), Group 2 dopants (e.g., Be, Mg, Ca, Sr, and/or Ba), Group 3 dopants (e.g., Sc, Y, and/or La), Group 4 dopants (e.g., Ti, Zr, and/or Hf), Group 5 dopants (e.g., V, Nb, and/or Ta), Group 6 dopants (e.g., Cr, Mo, and/or W), Group 7 dopants (e.g., Mn, Tc, and/or Re), Group 8 dopants (e.g., Fe, Ru, and/or Os), Group 9 dopants (e.g., Co, Rh, and/or Ir), Group 13 dopants (e.g., B, Al, Ga, In, and/or Tl), Group 14 dopants (e.g., C, Si, Ge, Sn and/or Pb), and/or Group 17 dopants (e.g., F, Cl, Br, and/or I). It should be understood that some electron transport layers may comprise exactly one type of dopant, and that some electron transport layers may comprise more than one type of dopant. In such cases, each dopant may independently be one of the dopants listed above.

When an electron transport layer comprises doped ZnO, the doped ZnO may comprise a variety of suitable amounts of dopants. In some embodiments, the doped ZnO comprises a dopant in an amount such that the ratio of the dopant to the Zn is greater than or equal to 0.001, greater than or equal to 0.002, greater than or equal to 0.005, greater than or equal to 0.0075, greater than or equal to 0.01, greater than or equal to 0.02, 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 0.2, greater than or equal to 0.3, or greater than or equal to 0.4. In some embodiments, the doped ZnO comprises a dopant in an amount such that the ratio of the dopant to the 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 0.2, 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 0.02, less than or equal to 0.01, less than or equal to 0.0075, less than or equal to 0.005, or less than or equal to 0.002. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 0.001 and less than or equal to 0.5). When an electron transport layer comprises ZnO comprising more than one dopant, each dopant may independently have a ratio to Zn in one or more of the ranges described above. Similarly, the ratio of the total amount of all dopants in the electron transport layer to the Zn may fall within one or more of the ranges described above.

It should also be understood that some embodiments may relate to electron transport layers comprising undoped ZnO. Such electron transport layers may further comprise doped ZnO or may lack doped ZnO. In some embodiments, a quantum dot light-emitting diode comprises one electron transport layer comprising doped ZnO and further comprises a second electron transport layer comprising undoped ZnO. If a quantum dot light-emitting diode comprises two electron transport layers comprising doped ZnO, the two layers may comprise the same type of doped ZnO and/or may comprise different types of doped ZnO (e.g., ZnO comprising different dopants, different combinations of dopants, and/or different amounts of one or more dopants). The electron transport layer(s) may have a variety of suitable morphologies. In some embodiments, an electron transport layer comprises one or more nanoparticles (e.g., one or more nanoparticles comprising doped ZnO, one or more nanoparticles comprising undoped ZnO). Such nanoparticles may be crystalline, amorphous, or partially crystalline and partially amorphous. When the electron transport layer comprises nanoparticles, it may further comprise one or more ligands surrounding and/or passivating the nanoparticles (e.g., capping 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, 1,10-decanedithiol, 3- mercaptopropionic acid, 4-mercaptobenzoic acid, benzoic acid, benzylamine, a silane (e.g., 3- aminopropyltriethoxy silane, triethoxysilylbutyraldehyde, 3 -isocyanatopropyl triethoxy silane,

3 -mercaptopropylthrimethoxy silane, 11-cyanoundecyltrimethoxysilane, 3- acryloxypropyltrimethoxysilane, triethoxysilylundecanal, n-(trimethoxysilylpropyl)ethylene diamine triacetic acid, 2- [methoxy(polyethyleneoxy)propyl]trimethoxy silane, and tetraethyl orthosilicate), and adenosine 5 '-monophosphate. When a quantum dot light-emitting diode comprises two or more electron transport layers comprising nanoparticles, each electron transport layer may independently comprise one or more of the ligands described above. When an 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, or less than or equal to 5 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 an electron transport layer comprises two or more types of nanoparticles, each type of nanoparticle may independently have a diameter in one or more of the ranges listed above and/or all of the nanoparticles in the electron transport layer together may have a diameter in one or more of the ranges listed above. Similarly, when a quantum dot light-emitting diode comprises two or more electron transport layers comprising nanoparticles, each electron transport layer may independently have the properties described in the preceding sentence.

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.

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.

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

EXAMPLES

The present invention 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. 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 undoped ZnO nanocrystals were sequentially deposited onto the TFB by spin coating at 2000 rpm for 60 s and 30 s, respectively. The average diameter of undoped ZnO

nanocrystals was less than 5 nm. The quantum dots were Cd-based core/shell colloidal nanocrystals with a green color. A1 electrodes (100 nm) were deposited onto the undoped 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

Three different quantum dot light-emitting diodes were prepared, each comprising an electron transport layer comprising ZnO nanocrystals doped with a different type of dopant. One quantum dot light-emitting diode comprised an electron transport layer comprising ZnO nanocrystals doped with 5 mol% Cs, one quantum dot light-emitting diode comprised an electron transport layer comprising ZnO nanocrystals doped with 10 mol% Mg, and one quantum dot light-emitting diode comprised an electron transport layer comprising ZnO nanocrystals doped with 5 mol% Ga. Each quantum dot light-emitting diode was fabricated by following the procedure described in the following paragraph.

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 doped ZnO nanocrystals were sequentially deposited onto the TFB by spin coating at 2000 rpm for 60 s and 30 s, respectively. The doped ZnO nanocrystals had an average diameter within 10% of that of the undoped ZnO nanocrystals of Comparative Example 1. The quantum dots were Cd-based core/shell colloidal nanocrystals with a green color. 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. 2 shows the current density as a function of voltage for Example 1 and Comparative Example 1. The solid lines with circles, triangles, and diamonds show data from Example 1 and the solid line with squares shows data from Comparative Example 1. Example 1 had a lower leakage current than that Comparative Example 1 and was much more stable than Comparative Example 1, while the total resistance of Example 1 was similar with

Comparative Example 1. This improvement may be due to the reduction of injection barrier between the A1 cathode and the ZnO electron transport layer due to the formation of ohmic contact between two materials. Ohmic contact between the A1 cathode and the doped ZnO electron transport layer indicates that the conduction band of the doped ZnO electron transport layer has an energy close to that of the conduction band of the quantum dot light- emitting layer. It is believed that this results in efficient injection of electrons in the doped ZnO electron transport layer into the transport band of the quantum dot light-emitting layer. Without wishing to be bound by any particular theory, it is believed that the Fermi energy in

ZnO can be extracted from the nominal doping density, according to E_F = E_t + kT In

where E is the intrinsic Fermi energy for ZnO; k is the Boltzmann constant; T is the temperature; and Nd/a is the nominal donor/acceptor doping density. The equation assumes that all the donor/acceptors are ionized, which is reasonable for the range of doping levels for the electron transport layer described in this Example. When the electron transport layer is doped to a sufficient degree, it is believed that the added electrons provided by the dopants and the associated decreased Fermi energy cause the electrons in the doped ZnO electron transport layer to be directly injected to the conduction band of the quantum dot light- emitting layer. It is believed that this direct injection advantageously traps interfacial defects present in the quantum dot light-emitting layer.

FIG. 3 shows the current efficiency (CE) as a function of luminance for the Example 1 and Comparative Example 1 and FIG. 4 shows external quantum efficiency (EQE) as a function of luminance for Example 1 and Comparative Example 1. The solid lines with circles, triangles, and diamonds show data from Example 1 and the solid line with squares shows data from Comparative Example 1. The CEs of Example 1 are much higher than that of

Comparative Example 1. In addition, the peak external quantum efficiencies of Example 1 are higher than that of Comparative Example 1. Since Example 1 and Comparative Example 1 have relatively low levels of leakage current, it is believed that the high CEs and high peak external quantum efficiencies of Example 1 result from the reduced Fermi energy and the resultant reduced injection barrier between the doped ZnO electron transport layer and quantum dot light-emitting layer.

FIG. 5 shows the T50 lifetime as a function of time for the Example 1 and Comparative Example 1. The solid lines with circles, triangles, and diamonds show data from Example 1 and the solid line with squares shows data from Comparative Example 1. The square, circular, and triangular symbols show data from these samples with doped ZnO nanocrystals with Cs, Mg, and Ga. The lifetimes of devices with doped ZnO electron transport layers are higher than that of device with undoped ZnO electron transport layer.

COMPARATIVE EXAMPLE 2

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 undoped ZnO nanocrystals were sequentially deposited onto the TFB by spin coating at 2000 rpm for 60 s and 30 s, respectively. The average diameter of the undoped ZnO nanocrystals was less than 5 nm. The quantum dots were Cd-based core/shell colloidal nanocrystals with a green color. A1 electrodes (100 nm) were deposited onto the undoped 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 2

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 doped with 5 mol% Rb were sequentially deposited onto the TFB by spin coating at 2000 rpm for 60 s and 30 s, respectively. The doped ZnO nanocrystals had an average diameter within 10% of that of the undoped ZnO nanocrystals of Comparative Example 2. The quantum dots were Cd-based core/shell colloidal nanocrystals with a green color. 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. 6 shows the current density as a function of voltage for Example 2 and Comparative Example 2. The solid line with circles shows data from Example 2 and the solid line with squares shows data from Comparative Example 2. Example 2 has a higher leakage current than that Comparative Example 2 and is much less stable than Comparative Example 2, while the total resistance of Example 2 is similar with Comparative Example 2. However, turn-on voltage of Example 2 is 0.4 V less than that of Comparative Example 2. This improvement may be due to the reduction of injection barrier between the A1 cathode and the ZnO electron transport layer due to the formation of an ohmic contact between the two materials. It is believed that the interface between quantum dot light-emitting layer and doped ZnO electron transport layer traps injected electrons.

FIG. 7 shows the current efficiency (CE) as a function of luminance for the Example 2 and Comparative Example 2 and FIG. 8 shows external quantum efficiency (EQE) as a function of luminance for Example 2 and Comparative Example 2. The solid line with circles shows data from Example 2 and the solid line with squares shows data from Comparative Example 2. The CE of Example 2 is much higher than that of Comparative Example 2. In addition, the peak external quantum efficiencies of Example 2 are higher than that of Comparative Example 2. Although Example 2 and Comparative Example 2 have relatively high levels of leakage current, it is believed that the high CEs and high peak external quantum efficiencies of Example 2 are due to the reduced Fermi energy and the resultant reduced injection barrier between the doped ZnO electron transport layer and the quantum dot light-emitting layer.

COMPARATIVE 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, 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 undoped ZnO nanocrystals were sequentially deposited onto the TFB by spin coating at 2000 rpm for 60 s and 30 s, respectively. The average diameter of undoped ZnO nanocrystals was less than 5 nm. The quantum dots were Cd-based core/shell colloidal nanocrystals with a green color. A1 electrodes (100 nm) were deposited onto the undoped 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 3

Two different quantum dot light-emitting diodes were prepared, each comprising an electron transport bilayer comprising a layer of silane ligand-capped ZnO nanocrystals and a layer of uncapped ZnO nanocrystals. One quantum dot light-emitting diode comprised an electron transport layer comprising silane ligand -capped ZnO nanocrystals doped with 5 mol% Mg (Example 3-1) and uncapped ZnO. The other quantum dot light-emitting diode comprised an electron transport layer comprising silane ligand-capped ZnO and uncapped ZnO nanocrystals doped with 5 mol% Mg (Example 3-2). Each quantum dot light-emitting diode was fabricated by following the procedure described in the following paragraph.

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 were deposited onto the TFB by spin coating at 2000 rpm for 60 s. Then, the electron transport bilayer was formed. First, silane-capped ZnO nanocrystals were spincoated onto the quantum dots at 3500 rpm for 30 s. Then, the resultant article was placed in an hotplate held at 70 °C for 30 minutes. Next, uncapped ZnO nanocrystals were spincoated onto the silane-capped ZnO nanocrystals. After these steps, the resultant article was placed in an hotplate held at 100 °C for 30 minutes. Both types of ZnO nanocrystals had an average diameters within 10% of that of the undoped ZnO nanocrystals of Comparative Example 3. The quantum dots were Cd-based core/shell colloidal nanocrystals with a green color. 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. 9 shows the current density as a function of voltage for Example 3-1, Example 3-2, and Comparative Example 3. The solid line shows data from Comparative Example 3 and the dashed line and dotted line show data from Example 3-1 and 3-2, respectively. Examples 3- 1 and 3-2 have higher electric tum-on voltages than Comparative Example 3 and lower total resistances than Comparative Example 3. These improvements may be due to the reduction of the injection barrier between the cathode and the electron transport layer(s) due to the formation of an ohmic contact between these layers. These improvements may also be due to the formation of an ohmic contact between the electron transport layers and the quantum dot light-emitting layer. It is believed that the ohmic contact between the electron transport layers and the quantum dot light-emitting layer is formed due to a low level of defects at the interface between the quantum dot light-emitting layer and the silane ligand-capped ZnO electron transport layer. It is also believed that the low layer of defects at this interface reduces the work function of silane ligand-capped ZnO electron transport layer by passivating the surface defects of ZnO with silicon and that ZnO/silica (core/shell) nanoparticles exhibit a stronger quantum confinement effect in comparison to uncapped ZnO nanoparticles.

FIG. 10 shows the external quantum efficiency (EQE) as a function of current density for Example 3-1, Example 3-2, and Comparative Example 3. The solid line shows data from Comparative Example 3 and the dashed line and dotted line show data from Example 3-1 and 3-2, respectively. The EQE of Example 3-2 is much higher than those of Comparative Example 3 and Example 3-1. In addition, the peak external quantum efficiency of Example 3-2 is higher than those of Comparative Example 3 and Example 3-1. It is believed that the slow efficiency droop and high peak external quantum efficiencies of Example 3-2 are due to improved charge balance in the quantum dot emission layer. It is also believed that the reduced number of defects at the interface between the electron transport bilayer and the quantum dot light-emitting layer , and the associated reduced work function of the electron transport layer comprising silane-capped nanoparticles comprising Mg-doped ZnO, cause this improved charge balance.

EXAMPLE 4

A quantum dot light-emitting diode comprising an electron transport bilayer comprising a layer of silane ligand-capped ZnO nanocrystals doped with 15 mol% Mg and a layer of uncapped ZnO nanocrystals was prepared. The procedure employed is described in the following paragraph. A PEDOT:PSS solution was filtered through a syringe filter (0.45 mhi) and then spin-coated onto an ITO-coated glass substrate at 3000 rpm for 60 s. After which, the resultant substrate was baked at 145 °C for 15 min. The PEDOT:PSS-coated substrate was then transferred into a nitrogen-filled globe box. Next, the material forming the hole transport layer was spin-coated onto the PEDOT:PSS-coated substrate at 2000 rpm for 30 s, which was then baked at 150 °C for 30 min. Cd-free InP quantum dots and doped ZnO nanocrystals were sequentially deposited onto the hole transport layer by spin coating at 4000 rpm for 60.

Then, the electron transport bilayer was formed. First, silane-capped ZnO nanocrystals doped with 15 mol% Mg were spincoated onto the quantum dots at 3500 rpm for 30 s.

Then, the resultant article was placed in an hotplate held at 70 °C for 30 minutes. Next, uncapped ZnO nanocrystals doped with 15 mol% Mg were spincoated onto the silane-capped ZnO nanocrystals. After these steps, the resultant article was placed in an hotplate held at 100 °C for 30 minutes. Both types of ZnO nanocrystals had an average diameters within 10% of that of the undoped ZnO nanocrystals of Comparative Example 3. The quantum dots were Cd-free core/shell colloidal nanocrystals with a red color. 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. 11 shows the current density as a function of voltage for Example 4. The solid line (tl.l) and the dashed line (t2.3) show data from Example 4 at measured 1 and 2 days after fabrication, respectively. Example 4 has a stable optical turn-on voltage of 1.7 V at tl.l and t2.3. FIG. 12 shows the external quantum efficiency (EQE) as a function of current density for the Example 4. The solid line (tl.l) and the dashed line (t2.3) show data from Example 4 measured 1 and 2 days after fabrication, respectively. The peak EQEs of Example 4 are 13.17 % at 276 Cd/m² for tl.l and 13.1 % at 264 Cd/m² for t2.3. It is believed that the stable efficiencies are due to improved charge balance in the quantum dot light-emitting layer due to a reduction in the amount of defects at the interface between the electron transport bilayer and the quantum dot light-emitting layer and due to a reduction in the work function of the electron transport bilayer.

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

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, wherein the electron transport layer comprises nanoparticles comprising ZnO doped with one or more dopants, and wherein the nanoparticles have a number average diameter of less than or equal to 5 nm; and a second electrode disposed on the electron transport layer.

2. The quantum dot light-emitting diode of claim 1, wherein at least one of the dopants is selected from the group consisting of Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Al, Ga, In, Si, Ge, and Sn.

3. The quantum dot light-emitting diode of any preceding claim, wherein a molar ratio of the one or more dopants to Zn in the hole transport layer is greater than or equal to 0.001 and less than or equal to 0.5.

4. The quantum dot light-emitting diode of any preceding claim, wherein the nanoparticles are crystalline.

5. The quantum dot light-emitting diode of any preceding claim, 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.

6. The quantum dot light-emitting diode of claim 5, wherein the quantum dot light- emitting diode further comprises a hole injection layer positioned between the first electrode and the hole transport layer.

7. The quantum dot light-emitting diode of any preceding claim, wherein the electron transport layer comprises a plurality of ligands surrounding the nanoparticles.

8. The quantum dot light-emitting diode of claim 7, wherein the plurality of ligands comprises a silane.

9. The quantum dot light-emitting diode of any preceding claim, further comprising a second electron transport layer.

10. The quantum dot light-emitting diode of claim 9, wherein the second electron transport layer comprises nanoparticles comprising ZnO.

11. A method of manufacturing a quantum dot light-emitting diode, comprising: assembling an electron transport layer with a first electrode, a second electrode, and a quantum dot light-emitting layer, wherein the electron transport layer comprises nanoparticles comprising ZnO doped with one or more dopants, and wherein the nanoparticles have a number average diameter of less than or equal to 5 nm.

12. The method of claim 11, wherein at least one of the dopants is selected from the group consisting of Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Al, Ga, In, Si, Ge, and Sn.

13. The method of any one of claims 11-12, wherein a molar ratio of the one or more dopants to Zn in the hole transport layer is greater than or equal to 0.001 and less than or equal to 0.5.

14. The method of any one of claims 11-13, wherein the nanoparticles are crystalline.

15. The method of any one of claims 11-14, 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.

16. The method of claim 15, wherein the quantum dot light-emitting diode further comprises a hole injection layer positioned between the first electrode and the hole transport layer.

17. The method of any one of claims 11-16, wherein the electron transport layer comprises a plurality of ligands surrounding the nanoparticles.

18. The method of claim 17, wherein the plurality of ligands comprises a silane.

19. The method of any one of claims 11-18, wherein assembling the electron transport layer with the first electrode, the second electrode, and the quantum dot light-emitting layer comprises performing a solution coating process.

20. The method of claim 19, wherein the solution coating process is selected from the group consisting of sol-gel coating, spin coating, printing, casting, stamping, dip coating, roll- to-roll coating, and spraying.

21. The method of any one of claims 19-20, wherein the electron transport layer is deposited by the solution coating process.

22. The method of any one of claims 19-21, wherein the quantum dot light-emitting layer is deposited by the solution coating process.

23. The method of any one of claims 19-22, wherein a hole transport layer is deposited by the solution coating process.

24. The method of claim 23, wherein the quantum dot light-emitting layer is deposited onto the hole transport layer.

25. The method of any one of claims 11-24, wherein the electron transport layer is deposited onto the quantum dot light-emitting layer.

26. The method of any one of claims 11-25, further comprising assembling a second electron transport layer with the electron transport layer, the first electrode, the second electrode, and the quantum dot light-emitting diode.

27. The method of claim 26, wherein the second electron transport layer comprises nanoparticles comprising ZnO.

28. The method of any one of claims 26-27, wherein the second electron transport layer is deposited by a solution coating process.

29. The method of claim 28, wherein the solution coating process is selected from the group consisting of sol-gel coating, spin coating, printing, casting, stamping, dip coating, roll- to-roll coating, and spraying.

30. The method of any one of claims 11-29, further comprising performing a thermal annealing step.

31. The method of claim 30, wherein the thermal annealing is performed at a temperature of greater than or equal to 70 and less than or equal to 200 °C.

32. The method of any one of claims 30-31, wherein the thermal annealing is performed in the presence of nitrogen, argon, helium, air, and/or oxygen.

33. A quantum dot light-emitting diode formed by the method of any one of claims 11- 32.