Electron transfer layer precursor and electron transfer layer for QLED

Abstract

Provided is a composition comprising ZnO-based nanoparticles, a polar organic solvent and a polymer, wherein the ZnO-based nanoparticles have the formula LixMgyMpZnzO, where 0 < x < 0.2; 0 < y+p < 0.2

Description

[0001] P101299 ELECTRON TRANSFER LAYER PRECURSOR AND ELECTRON TRANSFER LAYER FOR QLED FIELD OF THE INVENTION The present invention relates to a composition, the composition being a precursor for an electron transfer layer, a use of said composition, a method of manufacturing an electron transfer layer for a QLED, an electron transfer layer for a QLED, a QLED and a visual display unit. BACKGROUND OF THE INVENTION Due to their favourable band gap and electron transport properties, zinc oxide materials are used as electron transfer layers (ETLs) in pixels found in next generation display devices, such as in so-called QLED (quantum dot light emitting diode) displays. Pixels in next generation display devices typically comprise five or more layers including a quantum dot light emitting layer, an electron transport layer and a hole transport layer. Typically, each of said layers are about 50-150 nanometres thick. An important measure of the efficiency of a pixel in a display device is the external quantum efficiency (EQE). The EQE may be defined as the ratio of the number of photons emitted by the pixel to the number of electrons passing through the pixel. The external quantum efficiency may be regarded as a measure of how efficiently the device coverts electrons to photons. The external quantum efficiency is a measure of the whole pixel. However, all other factors being equal, EQE can be used to measure the effectiveness of the zinc oxide ETL. The highest reported EQE for an experimental QLED is around 20%. For commercial application, an external quantum efficiency of greater than 10% is desirable. Other factors, e.g. operational lifetime and working voltage, are also important considerations in preparing electron transfer layers. Heng Zhang et al., in J. Mater. Chem. C, 2019, 7, 2291-2298 discloses the use of a ZnMgO:PVP (polyvinylpyrrolidone) inorganic-organic hybrid ETL in an attempt to manufacture an improved ETL for a QLED. However, there remains a need to further improve P101299 ETLs for QLEDs, and in particular, there is a continued desire to improve the EQE of such QLEDs. SUMMARY OF THE INVENTION One aspect of the present disclosure is directed to a composition comprising ZnO- based nanoparticles, a polar organic solvent and a polymer, wherein the ZnO-based nanoparticles have the formula LixMgyMpZnzO, where 0 ≤ x ≤ 0.2; 0 ≤ y+p ≤ 0.25; z = 1 – 0.5x – y – 1.5p; and M is an element selected from gallium and / or aluminium; and wherein the polymer comprises polyvinylpyrrolidone (PVP) and / or poly(ethyl oxazoline) (PEO), the polymer having an average molecular weight, Mw, of from 25,000 to 1,300,000 Da. Another aspect of the present disclosure is directed to a method of manufacturing an electron transfer layer (ETL) for a QLED, the method comprising: providing a composition according the above aspect; spin coating the composition to provide a spin-coated composition; and annealing the spin-coated composition. Another aspect of the present disclosure is directed to the use of the composition of the above aspect in the manufacture of an electron transfer layer (ETL) for a QLED. Another aspect of the present disclosure is directed to an electron transfer layer (ETL) for a QLED comprising ZnO-based nanoparticles and a polymer, wherein the ZnO-based nanoparticles have the formula LixMgyMpZnzO, where 0 ≤ x ≤ 0.2; 0 ≤ y+p ≤ 0.25; z = 1 – 0.5x – y – 1.5p; and M is an element selected from gallium and / or aluminium; and wherein the polymer comprises polyvinylpyrrolidone (PVP) and / or poly(ethyl oxazoline) (PEO), the polymer having an average molecular weight, Mw, of from 25,000 to 1,300,000 Da. Another aspect of the present disclosure is directed to a QLED comprising an ETL according to the above aspect. Another aspect of the present disclosure is directed to a visual display unit comprising a QLED according to the above aspect. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the measured external quantum efficiencies of the QLEDs of Example 5. Figure 2 shows the measured voltages of the QLEDs of Example 5. P101299 DETAILED DESCRIPTION OF THE INVENTION The present invention seeks to tackle at least some of the problems associated with the prior art or at least to provide a commercially acceptable alternative solution thereto. In a first aspect, the present invention provides a composition comprising ZnO-based nanoparticles, a polar organic solvent and a polymer, wherein the ZnO-based nanoparticles have the formula LixMgyMpZnzO, where 0 ≤ x ≤ 0.2; 0 ≤ y+p ≤ 0.25; z = 1 – 0.5x – y – 1.5p; and M is an element selected from gallium and / or aluminium; and wherein the polymer comprises polyvinylpyrrolidone (PVP) and / or poly(ethyl oxazoline) (PEO), the polymer having an average molecular weight, Mw, of from 25,000 to 1,300,000 Da. Each aspect or embodiment as defined herein may be combined with any other aspect(s) or embodiment(s) unless clearly indicated to the contrary. In particular, any features indicated as being preferred or advantageous may be combined with any other feature indicated as being preferred or advantageous. Through a series of experiments, the inventor of the present invention has surprisingly found that varying the average molecular weight of the polymer in a composition according to the invention can have a significant influence on the EQE of a QLED comprising an ETL made using such a composition. The composition is typically a suspension and the polymer may typically be known as a dispersant or stabiliser. Such terms may be used interchangeably herein. The use of PVP as such a stabiliser has been investigated by Heng Zhang et al., for example, as discussed above. However, the impact of its molecular weight on stability and / or performance has not been investigated. The inventor has surprisingly found that an average molecular weight, Mw, of from 25,000 to 1,300,000 Da may result in a composition with significantly increased EQE performance in a resulting QLED. P101299 Without wishing to be bound by theory, it is thought that these unexpected effects may be due to the structure of the resulting layer that may be formed. For example, the packing of the layer of the resulting ETL may be improved. This may help to achieve lower surface roughness in the ETL, which is desirable. A Mw of below 25,000 Da may result in decreased EQE performance compared to a QLED made using the composition of the invention, for example. The inventor has found that increasing the Mw of the dispersant may increase the EQE of such a QLED. However, if the Mw of the polymer is too high, then the unexpected effects may not be observed or may be lessened. In addition to the beneficial effects on the EQE of a resulting QLED, the inventor has also found that the stability, i.e. shelf life, of the composition may be increased by use of the particular stabilisers of the invention. Ideally, such compositions should be stable for months. However, without the presence of such a stabiliser, the nanoparticles may crash out of the solvent after about three weeks, for example. Moreover, the nanoparticles may have excellent dispersion within the composition. Without wishing to be bound by theory, it is thought that this may be due to favourable interactions with the particular polymer(s) of the invention, and in particular in the solvents described herein. The composition is typically colourless or translucent, which is thought to be due to the excellent dispersion of the nanoparticles within the composition. The composition may therefore allow for the production of readily dispersible, in polar solvent, (doped) ZnO mono-size particles without the need for ultrasonic treatment. It is also noted that the peak EQE may degrade after about 3 weeks if no stabiliser is present. However, with the stabiliser of the invention, the peak EQE may be stable for over three months with the presence of the stabiliser. It has also been found that, surprisingly, PEO of similar Mw may also achieve stability and resulting EQE almost as good as or comparable to that of PVP. Advantageously, PVP and PEO are both non-toxic. The term “nanoparticle” as used herein takes on its usual meaning in the art. For example, a nanoparticle may be a particle that has a diameter ranging between about 1 and 100 nm in size, measured by TEM. P101299 The term “ZnO-based nanoparticles” as used herein encompasses nanoparticles comprising ZnO or doped ZnO. The ZnO can be doped with lithium, magnesium, gallium and / or aluminium, for example. The ZnO-based nanoparticles have the formula LixMgyMpZnzO, where 0 ≤ x ≤ 0.2; 0 ≤ y+p ≤ 0.25; z = 1 – 0.5x – y – 1.5p; and M is an element selected from gallium and / or aluminium. The presence of Li is not essential. However, the typical methods of manufacturing such nanoparticles may involve the use of lithium-based precursors. Accordingly, there is typically residual lithium present in the ZnO-based nanoparticles. It should be understood that other methods that do not result in the presence of lithium may also be used. The impact on performance of the residual lithium is not well understood. However, it is thought that the residual lithium does not have a significant impact on performance of the ZnO-based nanoparticles in an ETL. Other unavoidable trace elements may also be present in minor amounts if alternative precursors are used. Preferably, the composition consists essentially of the ZnO-based nanoparticles, the polar organic solvent and the polymer, and more preferably consists of the ZnO-based nanoparticles, the polar organic solvent and the polymer. The term “consists essentially of” as used herein may encompass that specific further components other than those listed can be present, provided that they do not materially affect the essential characteristics of the composition. The composition may be a precursor for an ETL of a QLED, for example. Preferably, the polymer is PVP and / or PEO. As is known in the art, the average molecular weight, Mw, is determined by the formula: where Niis the number of molecules of molecular mass Mi. The Mwmay be measured using known techniques. Preferably, the polymer has an Mwof from 25,000 to 500,000 Da, more preferably from 25,000 to 200,000 Da, still more preferably from 25,000 to 100,000 Da, yet still more preferably from 25,000 to 75,000 Da, yet still more preferably from 30,000 to 70,000 Da, yet P101299 still more preferably from 35,000 to 70,000 Da, yet still more preferably from 40,000 to 65,000 Da, even more preferably from 45,000 to 65,000 Da and even more preferably from 50,000 to 63,000 Da. Most preferably, the polymer has an Mwof about 58,000 Da. Such molecular weights may result in particularly advantageous stability and enhanced EQE properties, as described herein. Preferably, the polymer comprises PVP. More preferably, the polymer is PVP. PVP may provide the best stability and enhanced EQE properties. Preferably, p = 0 and y > 0. In other words, the ZnO-based nanoparticles are preferably doped with only magnesium and optionally lithium. ZnO doped with magnesium may result in the best performance in an ETL layer. In an alternative preferred embodiment, M is preferably aluminium. In a further alternative preferred embodiment, y = 0 and p > 0, preferably wherein M is aluminium. In other words, the ZnO-based nanoparticles are preferably doped with only aluminium and / or gallium, preferably aluminium, and optionally lithium. In some embodiments, 0 < x ≤ 0.2, 0.01 ≤ x ≤ 0.2, 0.05 ≤ x ≤ 0.15, 0.08 ≤ x ≤ 0.12, or a range made by a combination of any of these end-points. The amount of lithium is not particularly limited and may depend on the method of manufacture. Preferably, 0 < y+p ≤ 0.25, more preferably 0.01 ≤ y+p ≤ 0.25, still more preferably 0.05 ≤ y+p ≤ 0.25, yet still more preferably 0.10 ≤ y+p ≤ 0.20, even more preferably 0.15 ≤ y+p ≤ 0.20. It should be understood that, of course, if p = 0 for example, then 0 < y+p ≤ 0.25 becomes 0 < y ≤ 0.25. In other words, such formulations do not require both of y and p to be non-zero. Such amounts of dopant, and particularly of magnesium and / or aluminium, may result in the best EQE performance when used in an ETL layer. The EQE produced by un- doped ZnO nanoparticles may generally be less than doped ZnO nanoparticles. Thus, it is generally preferred that the ZnO-based nanoparticles are doped with magnesium and / or M, i.e. preferably y+p > 0. The skilled person may select suitable polar organic solvents. However, the polar organic solvent preferably comprises ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol, or mixtures of two or more thereof, more preferably ethanol and / or isopropanol. P101299 Preferably, the polar organic solvent is ethanol and / or isopropanol. Most preferably, the solvent is ethanol. Ethanol may provide the most stable composition, for example. Preferably, the weight ratio of ZnO-based nanoparticles to polymer is from 1:0.05 to 1:0.5, preferably from 1:0.15 to 1:0.4, more preferably from 1:0.3 to 1:0.4. It has been found that a higher wt.% of the polymer, e.g. PVP, that is included in the composition may also increase the optimised properties discussed above. Without wishing to be bound by theory, it is thought that such a weight ratio may optimise the interaction between the polymer and the ZnO-based nanoparticles in the composition, for example. For example, it may be particularly preferred if the total weight of PVP is about 30wt.% of the total weight of the ZnO-based nanoparticles in the composition. Preferably, the composition comprises from 0.1 to 10 wt.% of the ZnO-based nanoparticles, based on the total weight of the polar organic solvent, more preferably from 0.5 to 5 wt.%, still more preferably from 1 to 4 wt.%, even more preferably from 2 to 3 wt.%, and most preferably about 2.5 wt.%. Such a concentration of nanoparticles may be particularly suitable for the spin-coating process to produce the ETL, for example. Preferably, the ZnO-based nanoparticles have a mean particle diameter of from 1 to 20 nm, preferably from 1 to 10 nm. This may be measured using TEM and / or SEM, for example, using standard techniques. Thus, the mean particle diameter is typically the number mean particle diameter, i.e. the D[1,0]. The particle size distribution of the ZnO-based nanoparticles is preferably monodisperse. In other words, the ZnO-based nanoparticles are preferably substantially uniform in size, for example each of the particles have the same particle diameter ±10%. It is preferred that the mean particle diameter is small so that the resulting ETL may be smooth, i.e. with a low surface roughness. Since the ETL is thin, small particle diameters may be required for a smooth surface. In a further aspect of the present invention, provided is a method of manufacturing an electron transfer layer (ETL) for a QLED, the method comprising: providing a composition according to the first aspect; spin coating the composition to provide a spin-coated composition; and annealing the spin-coated composition. P101299 Typically, the polar organic solvent may be substantially evaporated off during the formation of the ETL. Suitable spin coating techniques are known in the art. For example, spin coating may include a procedure used to deposit uniform thin films onto flat substrates. Usually a small amount of coating material is applied on the centre of the substrate, which is either spinning at low speed or not spinning at all. The substrate is then rotated at speed up to 10,000 rpm to spread the coating material by centrifugal force (and to evaporate the solvent). A machine used for spin coating may be called a spin coater, or simply spinner. Spin coating may involve spin coating the composition at about 1000 to about 3000 rpm, such as about 2000 rpm, for example, for about 30 to about 90 seconds, such as about one minute, for example. Spin coating may also be known as centrifugal rubber mould casting. The method utilises inertia to produce castings from a rubber mould, for example. Suitable annealing techniques are also known in the art. For example, annealing may involve annealing on a hot plate at about 80 to about 120°C, such as about 100°C, for example, for about 5 to about 15 minutes, such as about 10 minutes, for example. Annealing the spin- coated composition typically forms the ETL, which can be incorporated into a QLED. Alternatively, on a larger scale, inkjet printing may be used to manufacture an ETL using the composition disclosed herein. In a further aspect of the present invention, provided is the use of the composition of the first aspect in the manufacture of an electron transfer layer (ETL) for a QLED. The skilled person is aware of suitable techniques to use such a composition. In a further aspect of the present invention, provided is an electron transfer layer (ETL) for a QLED comprising ZnO-based nanoparticles and a polymer, wherein the ZnO-based nanoparticles have the formula LixMgyMpZnzO, where 0 ≤ x ≤ 0.2; 0 ≤ y+p ≤ 0.25; z = 1 – 0.5x – y – 1.5p; and M is an element selected from gallium and / or aluminium; and P101299 wherein the polymer comprises polyvinylpyrrolidone (PVP) and / or poly(ethyl oxazoline) (PEO), the polymer having an average molecular weight, Mw, of from 25,000 to 1,300,000 Da. The preferred features and advantages described herein with reference to the first aspect apply equally to this aspect. It should be understood that the ETL of this aspect is the intended product of the precursor composition of the first aspect, for example. The ETL preferably has a surface roughness of 2 nm or less, for example 1.8 nm or less, 1.6 nm or less, 1.4 nm or less, or 1.2 nm or less. The surface roughness may be measured by, for example, atomic force microscopy. Smooth ETLs are particularly preferred in QLED technology. In a further aspect of the present invention, provided is a QLED comprising an ETL according to the above aspect. Such a QLED may demonstrate an improved EQE as described herein. In a further aspect of the present invention, provided is a visual display unit comprising a QLED according the above aspect. The invention will now be described in relation to the following non-limiting examples. Production of compositions Compositions according to the invention were produced according to the following examples. Example 1 - Preparation of nanoparticle 15 mole% Mg-doped ZnO from Zn / Mg acetate / hexane Zinc acetate dihydrate (0.012 moles, 2.63g, Aldrich 99.999%) and magnesium acetate tetrahydrate (0.45g, 15 mole%) were added to 120 ml of ethanol, and refluxed for 2.5 hours, a clear solution resulting. The solution was then allowed to cool to room temperature. P101299 Then lithium hydroxide (0.014 moles, 0.34g) was dissolved in 120 ml of ethanol, with ultrasonic treatment for about 15 minutes. This solution was then added to the zinc acetate / magnesium acetate tetrahydrate solution over 15 minutes, with vigorous stirring. No obvious change occurred. This was then left stirring at room temperature overnight (about 17 hours). No obvious change had occurred after this period. 240 ml of the solution was then made up to 1000 ml with n-hexane to precipitate the Mg-doped-ZnO, a pale milky solution resulting. The precipitate was allowed to settle out of the Mg-doped-ZnO / ethanol / hexane dispersion for two hours, and the supernatant liquor drained off. The remaining dispersion was placed in two 50 ml centrifuge tubes and centrifuged for 5 minutes at 7000 rpm. After decantation, the white product in the tubes was rinsed with dried ethanol (2ml x2) and 25 ml ethanol (dried over a molecular sieve) added to give a 2.5% wt. dispersion. The tubes were agitated to disperse the product, clear solutions resulting in both cases. The solutions were combined and 1.8 ml of 10% PVP (Mw = 25,000) in ethanol added. The composition of the ZnO-based nanoparticles was expected to be Li0.07Mg0.15Zn0.815O. Example 2 – preparation of nanoparticle 10 mole% Mg-doped ZnO from Zn / Mg acetate / hexane Zinc acetate dihydrate (0.01 moles, 2.195g, Aldrich 99.999%) and magnesium acetate tetrahydrate (0.228g, 10 mole%) were added to ethanol, and refluxed for 2.5 hours, a clear solution resulting. The solution was then allowed to cool to room temperature. Then lithium hydroxide (0.01 moles, 0.275g) was dissolved in 100 ml of ethanol, with ultrasonic treatment for about 15 minutes. This solution was then added to the zinc acetate / magnesium acetate tetrahydrate solution over 15 minutes, with vigorous stirring. No obvious change occurred. This was then left stirring at room temperature overnight (22.5 hours). No obvious change had occurred after this period. 200 ml of the solution was then made up to 900 ml with n-hexane to precipitate the Mg-doped-ZnO, a pale milky solution resulting. The precipitate was allowed to settle out of the Mg-doped-ZnO / ethanol / hexane dispersion for two hours, and the supernatant liquor drained off. The remaining dispersion was placed in two 50 ml centrifuge tubes, and P101299 centrifuged for 5 minutes at 7000 rpm. After decantation, the white product in the tubes was rinsed with dried ethanol (2ml x2) and ethanol (dried over a molecular sieve) added to give a 2.5% wt. dispersion. The tubes were agitated to disperse the product, clear solutions resulting in both cases. The solutions were combined and PVP (Mw = 25,000) and PVP (Mw = 58,000) were added: Tube 1: 0.75 ml of 10% PVP25,000 / EtOH per 10 ml of the dispersion of nanoparticles Tube 2: 0.75 ml of 10% PVP58,000 / EtOH per 10 ml of the dispersion of nanoparticles The composition of the ZnO-based nanoparticles was expected to be Li0.07Mg0.1Zn0.865O. Example 3 - preparation of nanoparticle 5 mole% Al-doped ZnO from Zn / acetate / hexane Zinc acetate dihydrate (0.01 moles, 2.195g, 99.999% Sigma-Aldrich) and Al(acac)3 (0.172g, 5 mole%) were added to 100 ml of ethanol. The solution was heated to boiling, and the solids dissolved, giving a clear solution. After 2.5 hours, the solution was allowed to cool. Lithium hydroxide (0.011 moles, 0.252g) in 100 ml ethanol was then added over 10 minutes. The LiOH was previously dissolved in ethanol by ultrasonic treatment for 15 minutes. This was then left stirring overnight for 16 hours. 200 ml of the solution was then made up to 900 ml with n-hexane to precipitate the Al- doped-ZnO, a pale milky solution resulting. The precipitate was allowed to settle out of the Al-doped-ZnO / ethanol / hexane dispersion for two hours, and the supernatant liquor drained off. The remaining dispersion was placed in two 50 ml centrifuge tubes, and centrifuged for 5 minutes at 7000 rpm. After decantation, the white product in the tubes was rinsed with dried ethanol (2ml x2) and ethanol (dried over a molecular sieve) added to give a 2.5% wt. dispersion. The tubes were agitated to disperse the product, clear solutions resulting in both cases. The solutions were combined and PVP (Mw = 58,000) was added: Tube 1: 0.75 ml of 10% PVP25,000 / EtOH per 10 ml of the dispersion of nanoparticles Tube 2: 0.75 ml of 10% PVP58,000 / EtOH per 10 ml of the dispersion of nanoparticles P101299 The composition of the ZnO-based nanoparticles was expected to be Li0.07Al0.05Zn0.89O. Example 4 - preparation of nanoparticle 20 mole% Mg-doped ZnO from Zn / Mg acetate / hexane Zinc acetate dihydrate (0.01 moles, 2.195g, Aldrich 99.999%) and magnesium acetate tetrahydrate (0.536g, 20 mole%) were added to ethanol, and refluxed for 2.5 hours, a clear solution resulting. The solution was then allowed to cool to room temperature. Then lithium hydroxide (0.011 moles, 0.299g) was dissolved in 100 ml of ethanol, with ultrasonic treatment for about 15 minutes. This solution was then added to the zinc acetate / magnesium acetate tetrahydrate solution over 15 minutes, with vigorous stirring. No obvious change occurred. This was then left stirring at room temperature overnight (22.5 hours). No obvious change had occurred after this period. 200 ml of the solution was then made up to 900 ml with n-hexane to precipitate the Mg-doped-ZnO, a pale milky solution resulting. The precipitate was allowed to settle out of the Mg-doped-ZnO / ethanol / hexane dispersion for two hours, and the supernatant liquor drained off. The remaining dispersion was placed in two 50 ml centrifuge tubes, and centrifuged for 5 minutes at 7000 rpm. After decantation, the white product in the tubes was rinsed with dried ethanol (2ml x2) and ethanol (dried over a molecular sieve) added to give a 2.5% wt. dispersion. The tubes were agitated to disperse the product, clear solutions resulting in both cases. The solutions were combined and PVP (Mw = 25,000) and PVP (Mw = 58,000) were added: Tube 1: 0.75 ml of 10% PVP25,000 / EtOH per 10 ml of the dispersion of nanoparticles Tube 2: 0.75 ml of 10% PVP58,000 / EtOH per 10 ml of the dispersion of nanoparticles The composition of the ZnO-based nanoparticles was expected to be Li0.07Mg0.2Zn0.765O. P101299 Testing of QLEDs Example 5 Bottom-emitting red Cd-free QLEDs were manufactured using a Mg0.15ZnO-based ETL with different molecular weight PVP and PEO as stabiliser polymers. The layer structure of the red QLEDs were as follows: Al (80 nm); ~50-80 nm ETL; ~15-20 nm red Cd-free QDs; 20 nm hole transfer layer (HTL); 40 nm PEDOT:PSS (poly(3,4- ethylenedioxythiophene):poly(styrene-sulfonate)); 100 nm indium tin oxide (ITO). The ETLs of each QLED had no PVP / PEO, Mw 25,000 PVP, Mw 58,000 PVP and Mw 50,000 PEO, respectively. The QLEDs were tested and the peak EQE and EQE at 10 mA / cm2were measured. The results are shown in Figure 1. The peak EQE value is the higher value for each QLED, represented by the square markers. The EQE at 10 mA / cm2 is the lower value for each QLED, represented by the circular markers. The best EQE performance is seen for the Mw 58,000 PVP. The voltages were also measured in the same conditions and the results are shown in Figure 2. The voltages show the so-called “driving voltages” that produce the peak EQE and give the current density of 10 mA / cm2, respectively. Example 6 Red Cd-free QLEDs were also manufactured using different Mg or M dopants in the ZnO nanoparticles of the ETL and also with different molecular weights of PVP. The QLEDs were tested and the peak EQE and EQE at 10 mA / cm2 were measured. The results are described in Table 1 below. The EQE values are given to the nearest 1%. Magnesium, aluminium and gallium were each used as dopants. M, y and p refer to the formula of the ZnO-doped nanoparticles described herein. Li may have been present in minor amounts due to the method of manufacturing the ETL. The gallium-doped sample contained no PVP / PEO. The voltages were also measured in the same conditions and the results are shown in Table 2. The voltages are given to the nearest 0.25 V. P101299 Table 1

[0002] P101299 Table 2 The dispersions for which EQEs were highest are Mg0.1ZnO (1367) + PVP 25k; Mg0.15ZnO + PVP 25k; Mg0.15ZnO + PVP 58k; Mg0.2ZnO + PVP 25k; Mg0.2ZnO + PVP 58k; and Al0.05ZnO + PVP 58k. Most of the driving voltages were below 5 V. The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in P101299 the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.

Claims

P101299 Claims 1. A composition comprising ZnO-based nanoparticles, a polar organic solvent and a polymer, wherein the ZnO-based nanoparticles have the formula LixMgyMpZnzO, where 0 ≤ x ≤ 0.2; 0 ≤ y+p ≤ 0.25; z = 1 – 0.5x – y – 1.5p; and M is an element selected from gallium and / or aluminium; and wherein the polymer comprises polyvinylpyrrolidone (PVP) and / or poly(ethyl oxazoline) (PEO), the polymer having an average molecular weight, Mw, of from 25,000 to 1,300,000 Da.

2. The composition of claim 1, wherein the polymer has an Mw of from 25,000 to 200,000 Da, preferably from 25,000 to 100,000 Da, more preferably from 30,000 to 70,000 Da.

3. The composition of claim 1 or claim 2, wherein the polymer has an Mwof from 45,000 to 65,000 Da.

4. The composition of any preceding claim, wherein the polymer comprises PVP.

5. The composition of any preceding claim, p = 0 and y > 0.

6. The composition of any of claims 1 to 4, wherein y = 0 and p > 0, preferably wherein M is aluminium.

7. The composition of any preceding claim, wherein 0.05 ≤ y+p ≤ 0.25, preferably wherein 0.10 ≤ y+p ≤ 0.20, more preferably 0.15 ≤ y+p ≤ 0.20.P101299 8. The composition of any preceding claim, wherein the polar organic solvent comprises ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol, or mixtures of two or more thereof, preferably ethanol and / or isopropanol.

9. The composition of any preceding claim, wherein the weight ratio of ZnO-based nanoparticles to polymer is from 1:0.05 to 1:0.5, preferably from 1:0.15 to 1:0.4, more preferably from 1:0.3 to 1:0.

4.

10. The composition of any preceding claim, wherein the composition comprises from 0.1 to 10 wt.% of the ZnO-based nanoparticles, based on the total weight of the polar organic solvent.

11. The composition of any preceding claim, wherein the ZnO-based nanoparticles have a mean particle diameter of from 1 to 20 nm, preferably from 1 to 10 nm.

12. The composition of any preceding claim, wherein 0 < x ≤ 0.2, and preferably 0.01 ≤ x ≤ 0.

2.

13. A method of manufacturing an electron transfer layer (ETL) for a quantum dot light emitting diode (QLED), the method comprising: providing a composition according to any of claims 1 to 12; spin coating the composition to provide a spin-coated composition; and annealing the spin-coated composition.

14. Use of the composition of any of claims 1 to 12 in the manufacture of an electron transfer layer (ETL) for a QLED.

15. An electron transfer layer (ETL) for a QLED comprising ZnO-based nanoparticles and a polymer, wherein the ZnO-based nanoparticles have the formula LixMgyMpZnzO, where 0 ≤ x ≤ 0.2; 0 ≤ y+p ≤ 0.25;P101299 z = 1 – 0.5x – y – 1.5p; and M is an element selected from gallium and / or aluminium; and wherein the polymer comprises polyvinylpyrrolidone (PVP) and / or poly(ethyl oxazoline) (PEO), the polymer having an average molecular weight, Mw, of from 25,000 to 1,300,000 Da.

16. The ETL of claim 15, wherein the polymer has an Mw of from 25,000 to 200,000 Da, preferably from 25,000 to 100,000 Da, more preferably from 30,000 to 70,000 Da.

17. The ETL of claim 15 or claim 16, wherein the polymer has an Mwof from 45,000 to 65,000 Da.

18. The ETL of any of claims 15 to 17, wherein the polymer comprises PVP.

19. The ETL of any of claims 15 to 18, wherein p = 0 and y > 0.

20. The ETL of any of claims 15 to 18, wherein y = 0 and p > 0, preferably wherein M is aluminium.

21. The ETL of any of claims 15 to 20, wherein 0.05 ≤ y+p ≤ 0.25, preferably wherein 0.10 ≤ y+p ≤ 0.20, more preferably 0.15 ≤ y+p ≤ 0.

20.

22. The ETL of any of claims 15 to 21, wherein the weight ratio of ZnO-based nanoparticles to polymer is from 1:0.05 to 1:0.5, preferably from 1:0.15 to 1:0.4, more preferably from 1:0.3 to 1:0.

4.

23. The ETL of any of claims 15 to 22, wherein the ZnO-based nanoparticles have a mean particle diameter of from 1 to 20 nm, preferably from 1 to 10 nm.

24. A QLED comprising an ETL according to any of claims 15 to 23.P101299 25. A visual display unit comprising a QLED according to claim 24.

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