WO2009103124A1

WO2009103124A1 - Semiconductor device including nanocrystals and methods of manufacturing the same

Info

Publication number: WO2009103124A1
Application number: PCT/AU2009/000197
Authority: WO
Inventors: Paul Mulvaney; Benjamin Mashford
Original assignee: The University Of Melbourne
Priority date: 2008-02-22
Filing date: 2009-02-20
Publication date: 2009-08-27

Abstract

The present invention relates generally to methods of manufacturing semiconductor devices with p-n junctions including nanocrystals, for instance, quantum-dot light emitting devices ('QD-LEDs'). The present invention also provides semiconductor devices such as, for instance, QD-LEDs which have been manufactured in an economic fashion in respect of time, cost and processing complexity while affording to the device high efficiency.

Description

SEMICONDUCTOR DEVICE INCLUDING NANOCRYST ALS AND METHODS OF MANUFACTURING THE SAME

Field of the Invention

Background of the Invention

Electronic or "charge carrying" devices, formed using semi-conductor materials and including p-n junctions, are commonly used in photovoltaics, diodes, resistors, capacitors, printable electronics and light-emitting devices (LEDs). While such semiconductor devices have been used for some time many devices suffer in terms of sustained operational effectiveness as well as manufacturing simplicity, particularly electronic devices comprising nano-sized crystals or 'dots' (i.e. nanocrystal activated devices). Take for instance quantum-dot based semiconductor devices such as quantum-dot light emitting devices (QD-LEDs) and in particular organic QD-LEDs. Over time the emission quality and lifetime of organic QD-LEDs tend to deteriorate. This is typically attributed to the degradation of the organic based luminescent materials used. For instance, US 2004/0023010 discloses organic QD-LEDs where the electron transport layer (ETL) (also known as the electron-injection layer) and the hole transport layer (HTL) (also known as the hole-injection layer) are presented in the form of organic based thin films. The active luminescent layer in these QD-LEDs is inorganic in nature. In these layers organic/inorganic interfacial defects arise over time because the organic based thin films are in physical contact with the luminescent layer. Ensuring the stability of such devices is additionally compromised over time because the organic film layers tend to degrade upon exposure to atmospheric air and/or moisture. To overcome such degradation the organic components of these QD-LEDs are often encapsulated. Also, during the fabrication of these organic QD-LEDs great expense and care is usually taken to ensure the manufacturing process is conducted in an oxygen and nitrogen free environment.

In an attempt to address the above problems nanocrystal - comprising semiconductor devices formed of inorganic materials have been postulated. For example, QD-LEDs composed entirely of inorganic materials (i.e., inorganic QD-LEDs) are disclosed in US 6,797,412 and US 2007/0170446.

US 2007/0170446 is generally directed to inorganic QD-LEDs which comprise an inorganic HTL layer and an inorganic ETL. In the examples, the inorganic HTL layer is deposited on either a (i) sapphire substrate by metal-organic chemical vapour deposition (MOCVD) or (ii) an indium tin oxide substrate (ITO) by vacuum deposition. The inorganic ETL layers in examples 1 and 3 were formed using e-beam evaporation (Example 1 , TiO₂) and vacuum deposition (Example 3, CdTe).

For the economical fabrication of inorganic semiconductor devices (e.g. QD-LEDs), it is desirable to avoid the use of techniques involving a vacuum chamber. The use of a vacuum chamber is necessary for each of the following processes commonly used in the semiconductor industry: chemical vapor deposition, sputtering, e-beam evaporation, molecular-beam epitaxy and atomic layer deposition (ALD); The slow and cumbersome nature of materials-processing within a vacuum chamber increases the cost of manufacturing and also places practical limits on both the physical dimensions and the types of devices that can be made.

Accordingly, there is a need for semiconductor devices which can offer tunable emission wavelengths which can be fabricated using efficient process methodology wherein said devices are comprised of photostable and chemically robust materials. Summary of Invention

The present invention provides a method of manufacturing an inorganic semiconductor device including nanocrystals which comprises an inorganic electron transport layer (ETL), an inorganic semiconductor nanocrystal layer, and an inorganic hole transport layer (HTL), said method comprising the steps of forming the inorganic electron transport layer, inorganic semiconductor nanocrystal layer and inorganic hole transport layer by wet- chemical deposition processes.

In a further aspect the invention provides an inorganic semiconductor device including nanocrystals which comprises an inorganic electron transport layer (ETL), an inorganic semiconductor nanocrystal layer, and an inorganic hole transport layer (HTL), wherein all three layers are prepared by wet-chemical deposition processes.

The invention is partly based on the discovery that the three main component layers of inorganic semiconductor devices including nanocrystals can all be efficiently and more simply made by inorganic, wet-chemical deposition processes.

Brief Description of the Drawings

Figure 1 : Schematic view of a QD-LED device according to the invention.

Figure 2: Schematic view of an NiO based QD-LED device according to one embodiment of the invention.

Figure 3: Schematic view of an NiO based QD-LED device according to another embodiment of the invention.

Figure 4: Schematic view of an NiO based QD-LED device according to yet another embodiment of the invention. Figure 5: Schematic view of an NiO based QD-LED device according to yet another embodiment of the invention.

Figure 6: Graph showing absorption profile of a NiO based thin film prepared in accordance with an embodiment of the invention.

Figure 7: Graph showing the current- voltage behaviour of a device according to Example 1.

Figure 8: Graph showing the spectrum of the electroluminescence measured from the device according to Example 1.

Figure 9: Graph showing the spectrum of the photo luminescence measured from the device according to Example 1.

Detailed Description of the Invention

A semiconductor nanocrystal is used in the semiconductor device according to the present invention. The terms "semiconductor nanocrystal", "semiconductor nanoparticle" and "quantum dot" may be used herein interchangeably to refer to any small crystal or particle of a semiconducting material with a diameter less than lOOnm, and preferably less than 20nm. In any case, the particle, diameter is such that the emission properties are functions of the particle's size and shape. The size of the crystals or particles used in the semiconductor nanocrystal layer according to the present invention is a feature of this invention and is distinguished from known devices involving phosphor particles and other emissive layers made of substantially larger particles, whose emission properties are not strongly affected by the phosphor particle size.

One of the unique properties of semiconductor nanocrystals is that the emission from the crystals can be adjusted by variations in the size of the manufactured particles. In some cases further enhancements are possible by preparing the nanoparticles or nanocrystals in the form of rods or wires, by coating them with a second layer or by a combination of the above. Such modified or enhanced nanocrystals are also encompassed by the present invention. This is irrespective of the nomenclature assigned to these materials. For example, quantum rods and quantum wires denote the same type of materials as nanorods and nanowires respectively.

It is further found that semiconductor nanocrystals can be grown which exhibit tetrapodal or other complex geometries. Such nanowires, nanopods or other designated nanocrystals are considered germane and are envisaged in this invention, provided that one or more dimensions of said nanoparticles or nanocrystals is less than lOOnm, and preferably less than 20nm.

As used herein, reference to a "sol-gel process" means a process, which utilises wet- chemical techniques starting from a chemical solution that produces colloidal particles (or sols). Preferably, the sol-gel processes according to the present invention utilise metal alkoxides or metal chlorides, which undergo hydrolysis and polycondensation reactions to form a colloid (i.e., a system composed of solid particles dispersed in a solvent). The sol continues on to form an inorganic network containing a liquid phase and gel wherein metal-oxo (M-O-M) or metal-hydroxy (M-OH-M) polymers are formed in solution (referred to herein as 'the precursor sol'). A drying process serves to remove the liquid phase from the gel thus forming a porous material. A thermal treatment (annealing) is often conducted in order to favour further polycondensation and enhance mechanical, optical or electrical properties.

As used herein reference to "colloidal nanocrystal film" means a material which is also deposited from colloidal particles suspended in a solution, but which is distinct from a sol- gel material in that the particles do not form a gel network during the drying process. The particles form a porous solid but retain many of their individual characteristics, as required for example, for retaining the desired emission properties of the quantum dot (QD) layer. The deposition processes to form the layers of the inorganic semiconductor device of the present invention are all based on either of the two above described methodologies, which are collectively referred to herein as "wet-chemical deposition". Deposition of the precursor sol may occur on a substrate to form a layered film by any of the following techniques familiar to those in the field: dip-coating, spin-coating, screen printing, blade coating, brushing, rotational coating, spraying, drop casting or printing. The preferred method of film deposition depends on the size and type of the substrate being used and also on the properties of the particular sol being deposited. The drying and annealing processes to form the film as described above are thus typically carried out once deposition is completed.

The preferred processing parameters of each layer of the present invention are optimised to maximise conduction of charge through the device and at the same time preserve the emissive properties of the active semiconductor layer. Accordingly, the deposition and annealing of all layers subsequent to deposition of the active layer must be performed at processing temperatures that do not degrade the underlying emissive colloidal nanocrystal film.

Preferred processing ranges for each of the HTL, active semiconductor layer and ETL are as follows:

The HTL is deposited via wet-chemical deposition which includes one or more heating stages. The precursor sol for the HTL material is formed by dissolving one or more metal salts into a liquid solution to a concentration of between 0.01 and 1.0 M concentration, preferably, 0.1-0.8M, and more preferably 0.3-0.6M. Various other chemical additives may be used to enhance stability of the solution or to improve the quality of the film during deposition. After deposition, the film is heated at temperatures high enough to promote formation of a p-type material with high carrier concentration. The required temperature for this annealing is in the range of 300 - 1000° C, preferably 300-600 ⁰C and even more preferably 400-500 ⁰C. The preferred HTL is one which exhibits high transparency (greater than 70%) over the visible wavelength region and which exhibits an electrical resistivity of less than 100 Ω cm.

In an embodiment the invention also provides a method of manufacturing an inorganic semiconductor device including nanocrystals which comprises an inorganic electron transport layer (ETL), an inorganic semiconductor nanocrystal layer, and an inorganic hole transport layer (HTL)₅ said method comprising:

(i) forming the HTL by wet-chemical deposition including the steps of: (a) preparing a precursor sol by dissolving one or more metal salts into a liquid solution to a concentration of 0.01 to 1.0M;

(b) depositing the precursor sol onto a surface; and

(c) heating the deposited precursor sol for a time and under conditions sufficient to form the HTL; (ii) forming the inorganic semiconductor nanocrystal layer by wet-chemical deposition; and (iii) forming the ETL by wet-chemical deposition.

The semiconductor active layer is also deposited via wet-chemical deposition, which includes a heating stage at a temperature that allows evaporation of the solvent but which is insufficient to physically or chemically degrade the semiconductor nanocrystal components. The preferred temperature for this heating is in the range of 50 - 250 ⁰C, even more preferably 50 - 150 ⁰C and still even more preferably 50 - 100 ⁰C.

The ETL is also deposited via wet-chemical deposition, following which there is included one or more heating stages. The precursor sol for the ETL material is formed by dissolving one or more metal salts into a liquid solution to a concentration of between 0.01 and 1.0 M concentration, preferably, 0.1-0.8M, and more preferably 0.3-0.6M. Various other chemical additives may be used to enhance stability of the solution or to improve the quality of the film during deposition. Alternatively, the precursor sol consists of nanoparticles (formed of the intended ETL material) dispersed in solution. After deposition, the film is heated to promote evaporation of the solvent and optionally cause aggregation of the deposited colloidal material. The preferred temperature for this heating is in the range of 50 - 250° C, even more preferably 50 - 150 ⁰C and still even more preferably 50 - 100 ⁰C. Preferably the heating times are from 2-10 minutes.

The preferred ETL is one which exhibits an electrical resistivity of less than 100 Ω cm.

In an embodiment the invention also provides a method of manufacturing an inorganic semiconductor device including nanocrystals which comprises an inorganic electron transport layer (ETL), an inorganic semiconductor nanocrystal layer, and an inorganic hole transport layer (HTL), said method comprising:

(i) forming the inorganic HTL by wet-chemical deposition;

(ii) forming the inorganic semiconductor nanocrystal layer by wet-chemical deposition; and (iii) forming the inorganic ETL by wet-chemical deposition including the steps:

(a) preparing a precursor sol by dissolving one or more metal salts into a liquid solution to a concentration of 0.01 to 1.0M;

(b) depositing the precursor sol onto a surface; and

(c) heating the deposited precursor sol for a time and under conditions sufficient to form the ETL.

(i) forming the inorganic HTL by wet-chemical deposition;

(ii) forming the inorganic semiconductor nanocrystal layer by wet-chemical deposition; and

(iii) forming the inorganic ETL by wet-chemical deposition including the steps: (a) preparing a precursor sol by dispersing one or more nanoparticles in a solution; (b) depositing the precursor sol onto a surface; and

(b) depositing the precursor sol onto a surface; and

(c) heating the deposited precursor sol for a time and under conditions sufficient to form the HTL; (ii) forming the inorganic semiconductor nanocrystal layer by wet-chemical deposition; and (iii) forming the inorganic ETL by wet-chemical deposition including the steps:

(a) preparing a precursor sol by dissolving one or more metal salts into a liquid solution to a concentration of 0.01 to 1.0M; (b) depositing the precursor sol onto a surface; and

(i) forming the HTL by wet-chemical deposition including the steps of:

(b) depositing the precursor sol onto a surface; and (c) heating the deposited precursor sol for a time and under conditions sufficient to form the HTL;

(d) preparing a precursor sol by dispensing one or more nanoparticles in a solution;

(e) depositing the precursor sol onto a surface; and

(f) heating the deposited precursor sol for a time and under conditions sufficient to form the ETL.

The method according to the present invention as illustrated for instance in the above embodiments, includes forming the semiconductor nanocrystal layer on the HTL and subsequently forming the ETL on the semiconductor nanocrystal layer. As an alternative the method may also involve first forming the semiconductor nanocrystal layer on the ETL and subsequently forming the HTL on the semiconductor nanocrystal layer. Accordingly, in the embodiments referred to above it will be appreciated that reference to a "surface" includes a preformed layer of the device or some other substrate surface.

The material which may be used in the HTL is composed of an inorganic material capable of transporting holes even in an amorphous state, and is in particular selected from the group comprising NiO, Cu₂O, SrCu₂O, and p-ZnO or any combination of these including variations with chemical doping. In relation to conductive quality and optical transparency (in respect of the resultant emitted light from a QD-LED for instance) it has been found that NiO is a particularly preferred inorganic material for forming the HTL layer in respect of the methods of the present invention which utilise wet-chemical processes.

Accordingly, the present invention also provides an inorganic semiconductor device including nanocrystals which comprises an inorganic electron transport layer (ETL), an inorganic semiconductor nanocrystal layer, and a NiO based hole transport layer (HTL), wherein each layer has been prepared by a wet-chemical deposition process. As referred to above "NiO based" means that the HTL layer may consist essentially of NiO or comprise NiO and at least one other inorganic material capable of transporting holes as referred to above, or comprise NiO which has been doped. Suitable dopants include lithium (typically around 1% of atomic composition). NiO is especially preferred for use in the HTL because of the suitability of its valence band energy level, electrical conductivity and the high degree of optical transparency exhibited across the visible region.

The material which may be used in the inorganic ETL is composed of an inorganic material capable of transporting electrons even in an amorphous state, and in particular an inorganic metal oxide, sulphide or selenide selected from the following n-type semiconductor materials: ZnO, TiO₂, ZnS, ZnSe, ZrO₂, SiO₂, SnO₂, Ta₂O₅, In₂O₃, WO₃,

Nb₂O₅ or any combination of these including variations with chemical doping. However, other metal oxides, oxyhydroxides and hydroxides or mixtures thereof may also be suitable.

Preferably the inorganic material used in the inorganic ETL is ZnO, which includes combinations of ZnO with one or more other suitable n-type semiconductor materials as disclosed above, or ZnO which has been doped. Suitable dopants include Al, Ga and In. ZnO is especially preferred for use in the ETL because of the suitability of its conduction band energy level, electrical conductivity and the low temperature at which a conductive, colloidal nanocrystal film may be formed.

Accordingly, in an even more preferred embodiment the present invention provides an inorganic semiconductor device including nanocrystals which comprises a ZnO based inorganic electron transport layer (ETL), an inorganic semiconductor nanocrystal layer, and a NiO based hole transport layer (HTL), wherein each layer has been prepared by a wet-chemical deposition process. In another embodiment, the ETL and HTL are deposited from a solution wherein some condensation has already taken place. In this solution, the metal oxide is already in a polymeric, oligomeric or colloidal state prior to deposition.

In yet another embodiment, the sol precursor solution is heated prior to deposition to ensure the formation of said colloidal particles, whereby the colloid particles in the precursor solution that comprise the ETL or HTL are crystalline prior to deposition.

It is a further feature of this invention that the dispersion of the emissive nanocrystals in a sol-gel layer enables other charge carrying devices to be prepared by wet-chemical deposition technologies. For example, a further embodiment of this invention is a cell capable of separating charge carriers created by solar insolation or other electromagnetic radiation absorbed by the cell. Thus photovoltaic devices are conceived which are prepared by sol-gel methods and comprising nanocrystals embedded in a wide band gap semiconductor matrix.

It is recognized that by the emission of tunable light using a wet-chemical deposited LED that near infra-red and infra-red light may be emitted. This is useful in other applications such as lamps, signalling devices, communication devices, alarms, sensors and actuators in which the nanocrystals emit radiation of a specified wavelength upon application of an electronic input signal.

It is further envisaged that the emission from this wet-chemical deposited LED may consist of light emitted by just one nanocrystal. The light emitted from a single photon source has distinctive features. It is a property of the present invention that such a single photon source may be realized through a wet-chemically processed device incorporating semiconductor nanocrystals.

It is further envisaged that the light emitted by the single nanocrystal can be regulated or modulated by the electrical voltage applied to the device. This feature is fully recognized within this invention. It is a further embodiment that this light emitting structure may be part of a larger functional structure such as a circuit.

It is also recognized that the semiconductor device embodied in this invention may be printed or produced on a continuous basis as part of a larger structure or device. For example, an inorganic QD-LED device as prepared by the methods of the present invention may be printed using a roll-to-roll process, enabling large scale production of said LEDs.

In particular embodiments each layer forming the device is characterised by low levels of residual water and/or solvent and more preferably each layer is prepared substantially free of residual water and/or solvent. Those skilled in the art may appreciate the advantages conferred by having low levels of residual water and/or solvent in respect of, for instance, minimising electrochemical corrosion and hence maintaining high quantum efficiency over the operational lifetime of the device.

It is a feature of current QD-LED structures that the nanocrystals are deposited onto an ETL or HTL from a solution and then dried to form a film of aggregated particles. This feature is indeed common to all extant devices and inventions.

This feature has clear drawbacks in that it is difficult to control the aggregated state of the particles in terms of separation and packing density. It is not possible to prevent pinholes forming, and there are inevitably regions of different particle film thickness. Furthermore such an aggregated state does not permit a well defined resistance to the passage of charge carriers necessary for the efficient operation of the device.

In a further embodiment of this invention, the method of the present invention may include the step or steps of embedding the particles in a precursor solution and depositing the solution containing the particles by any of the methods previously described, such that the particles are dispersed homogeneously in a matrix of a semiconducting matrix. This methodology is illustrated by the following prophetic examples:

Example A The nanocrystals are dispersed in an ethanolic solution containing ZnO nanocrystals or Zn acetate. The solution is spin coated and dried then annealed. The nanocrystals are dispersed in a thin film of ZnO.

Example B The nanocrystals are dispersed in an ethanolic solution containing ZnS nanocrystals or Zn acetate and thiourea. The solution is spin coated and dried then annealed. The QDs are dispersed in a thin film of ZnS.

The advantage of this wet-chemical synthesis of the active layer over existing methods is evident. The QDs are embedded in a homogeneous environment. Their concentration and the film thickness can be regulated. This new method provides for a pinhole free, crack- free layer whose resistivity can be controlled and in which the nanocrystals are isolated from the deleterious effects of the environment, including oxygen and water.

Example C

The conductivity of the matrix can be modified during the creation of the active layer. The nanocrystals are dispersed in an ethanolic solution containing ZnO nanocrystals doped with In or Al or the solution contains Zn acetate and an indium or aluminium salt. The solution is spin coated and dried then annealed. The nanocrystals are dispersed in a thin film of In or Al doped ZnO.

Accordingly, in another embodiment the present invention provides a method of manufacturing an inorganic semiconductor device including nanocrystals which comprises an inorganic electron transport layer (ETL), an inorganic semiconductor nanocrystal layer, and an inorganic hole transport layer (HTL), said method comprising:

(i) forming the inorganic ETL by a wet-chemical deposition process; (ii) forming the inorganic semiconductor nanocrystal layer by a wet-chemical deposition process which includes the steps: (a) embedding nanoparticles in a precursor solution comprising a semiconducting material; and (b) depositing the precursor solution from step (a) onto a surface such that the nanoparticles are dispersed homogenously in a matrix of the semiconducting material; and (iii) forming the inorganic HTL by a wet-chemical deposition process.

Figure l is a schematic view showing a preferred embodiment of semiconductor device of the present invention in the form of an inorganic QD-LED. As shown, an inorganic QD- LED according to the present invention may have a structure including a first electrode 1 as an anode which is formed on a substrate 6, an inorganic HTL 3, a semiconductor nanocrystal layer 2, an inorganic ETL 4 and a second electrode 5. When a voltage is applied to the first and second metal electrodes 1 and 5, holes are injected into the HTL 3 from the first electrode, while electrons are injected into the ETL 4 from the second electrode 5. When the holes and electrons are brought into contact with each other in the semiconductor nanocrystals of the semiconductor nanocrystal layer 2, they bind to form excitons, which then recombine, and emit light. The light may be in the visible, UV or near IR region. It will be appreciated that the wavelength of the emitted light may be tunable depending on the size of the quantum dots and the nature or make-up of the semiconductor nanocrystal layer.

Thus the semiconductor nanocrystals used in the semiconductor nanocrystal layer may be selected from any known quantum dot material which has a quantum confinement effect due to size. Suitable semiconductor nanocrystals may be made from ZnS, CdS, CdSe, CdTe, ZnSe, ZnTe₅ HgS, HgSe, HgTe, GaN, GaP, GuAs, InP, InAs₅ PbS, PbSe, PbTe and nanocrystals including Si and Ge. Other suitable nanocrystals include mixtures of nanocrystals. Other suitable nanocrystals include nanocrystals which emit light and which have different shapes, such as rods or tetrapods. Furthermore, it is envisaged that the nanocrystals include alloys of two or more types of material. Examples include but are not limited to: CdS/CdSe, ZnS/ZnSe, CdS/ZnS, and CdSe/CdTe. It is also possible to tune the emission of a nanocrystal by doping. Examples include the addition of Te to CdSe and Mn ions to CdS or CdSe. Such modified nanocrystals are also considered suitable in the present invention.

Using the wet-chemical processes described herein the nanocrystals may be presented in the semiconductor nanocrystal layer in advantageous forms and arrangements. For instance, the nanocrystals may be presented in the semiconductor nanocrystal layer in the form of core/shell structures such as: CdSe/ZnS, CdSe/ZnSe, CdTe/ZnS, CdTe/ZnSe, CdSe/CdS, CdS/ZnS, CdS/ZeSe, InP/ZnS and PdSe/ZnS, wherein the shell is formed of a semiconductor material having a wider band gap.

For instance, Figure 2 shows an embodiment where the inorganic QD-LED comprises a semiconductor nanocrystal layer 7 where the nanocrystals are of the shell/core type which have been deposited (via a wet-chemical deposition process) onto 3 to form a luminescent thin film.

As an alternative, Figure 3 shows an embodiment where the inorganic QD-LED comprises a semiconductor nanocrystal layer 8 where the nanocrystals are of the shell/core type which are embedded into a transparent nanocrystal matrix which has been deposited (via a wet-chemical deposition process) onto 3 to form a luminescent thin film.

As a further alternative, Figure 4 shows an embodiment where the inorganic QD-LED comprises a semiconductor nanocrystal layer 10 where the nanocrystals are of the shell/core type and are arranged between and in physical contact with two layers 9 and 11 of semiconductor material having a wider band gap. The layers 9 - 11 are all deposited via a wet-chemical deposition process. As a still further preferred embodiment, Figure 5 shows an embodiment wherein the inorganic QD-LED comprises a semiconductor nanocrystal layer 12 wherein the nanocrystals are dispersed in a wide band gap semiconductor material to form a homogeneous distribution of the nanocrystals within the wide band gap semiconductor matrix. This homogeneous layer is also deposited via a wet-chemical deposition process.

It will be appreciated from the figures that the inorganic QD-LEDs of the present invention may comprise additional functional features including electrodes, transparent substrates, and conductive protective layers. For instance, in Figures 1 to 5, 5 represents a metal electrode (cathode) which may be selected from a selection of low- work function metals, including Al, In Mg, Ag, Ca, Li, Cs or alloys thereof. Also, as shown in the Figures, 1 also represents a metal electrode (anode) which may be selected from ITO (Indium Tin Oxide), IZO (Indium Zinc Oxide), Ni, Pt, Au, Ag, Ir, Pd and oxides thereof. Also, from the Figures it is contemplated that the HTL 3 is to be deposited (using a wet-chemical deposition process) on a substrate 1. The substrate may be selected from glass, quartz, silicon, GaN, GaAs, sapphire, or other smooth material.

Certain embodiments of the invention will now be described with reference to the following examples which are intended for the purpose of illustration only and are not intended to limit the scope of the generality hereinbefore described.

Examples

Example 1 - Fabrication of Electroluminescent Diode containing a wet-chemical deposited ETL, OD layer and a wet-chemical deposited HTL

The QD-LED according to this example is represented by Figure 2 wherein 1 is an ITO layer, 3 is a NiO based HTL layer, 7 is the luminescent QD layer of CdSe/ZnS (core/shell type), 4 is a ZnO ETL layer, 5 is metallic Ag, and 6 is a glass substrate. Fabrication Process:

1. A NiO sol-gel precursor solution is made by dissolving Nickel Acetate in propanol to a concentration of 0.4 M, along with an equimolar amount of diethanolamine. The sol- gel precursor is deposited onto an ITO coated glass substrate via spin-coating at 3000 rpm for 10 sees. The device is then subjected to annealing in air at a temperature of 425° C, forming a transparent HTL layer with a thickness of 50 nm.

2. A solution consisting of colloidal CdSe/ZnS core-shell nanocrystals capped with 5- aminopentanol (to render them soluble in alcoholic solvents) dispersed in propanol is prepared. This solution is then deposited on the NiO layer via spin-coating at 3000 rpm for 10 sees. The device is then heated at 100° C for 5 minutes under nitrogen to evaporate the solvent, forming a luminescent thin film with a thickness of 35 nm.

3. A solution consisting of colloidal ZnO nanocrystals dispersed in ethanol (concentration 0.1 M) is prepared via a low temperature base-catalyzed reaction well described in the literature. The nanocrystals were washed twice via repeated precipitation in hexane before being redispersed in fresh ethanol. The particle diameter as measured by electron microscopy was between 3-8 nm. The solution was deposited on the luminescent QD layer via spin-coating at 3000 rpm for 10 sees. The device is then heated at 100° C for 5 minutes under nitrogen, forming an ETL with a thickness of 40 nm.

4. Metallic silver is then deposited on the ZnO layer through a patterned mask to form the device cathode. Example 2 - Fabrication of Electroluminescent Diode containing a wet-chemical deposited ETL, QD-embedded wet-chemical deposited layer and a wet-chemical deposited HTL

The QD-LED according to this example is represented by Figure 3 wherein 1 is an ITO layer, 3 is a NiO based HTL layer, 8 is the luminescent QD layer of CdSe/ZnS (core/shell type), 4 is a ZnO ETL layer, 5 is metallic In, and 6 is a glass substrate.

Fabrication Process:

1. A NiO sol-gel precursor solution was made by dissolving Nickel Acetate in propanol to a concentration of 0.4 M, along with an equimolar amount of diethanolamine. The sol- gel precursor 5 was deposited onto an ITO coated glass substrate via spin-coating at 3000 rpm for 10 sees. The device was then subjected to annealing in air at a temperature of 425° C, forming a transparent HTL layer with a thickness of 50 nm.

2. A ZnS sol-gel precursor solution was made by dissolving Zinc Acetate and thiourea in a propanol/water mixture (ratio 6:1) to a concentration of 0.1 M. To this solution, QDs dispersed in propanol are added and the combined solution was deposited on the NiO layer via spin-coating at 3000 rpm for 10 sees. The device was then heated at 100° C for 5 minutes under nitrogen to evaporate the solvent, forming a luminescent thin film with a thickness of 100 nm. The layer thus formed consists of colloidal QDs homogenously dispersed into a transparent, ZnS matrix.

3. A solution consisting of colloidal ZnO nanocrystals dispersed in ethanol (concentration 0.1 M) was prepared via a low temperature base-catalyzed reaction well described in the literature. The nanocrystals were washed twice via repeated precipitation in hexane before being redispersed in fresh ethanol. The particle diameter as measured by electron microscopy was between 3-8 nm. The solution was deposited on the luminescent QD-ZnS layer via spin-coating at 3000 rpm for 10 sees. The device was then heated at 100° C for 5 minutes under nitrogen, forming an ETL with a thickness of 40 nm. 4. Metallic silver was then deposited on the ZnO layer through a patterned mask to form the device cathode.

Example 3 - Fabrication of Electroluminescent Diode containing a wet-chemical deposited ETL, 1^st wide-bandgap sol-gel layer, QD layer, 2^nd wide-bandgap sol-gel layer and wet-chemical deposited HTL

The QD-LED according to this example was represented by Figure 4 wherein 1 was an ITO layer, 3 was a NiO based HTL layer, 9 represents the luminescent QD layer of CdSe/ZnS (core/shell type), 10 and 11 represent ZnS layers, 4 was a ZnO ETL layer, 5 was metallic Ag, and 6 was a glass substrate.

Fabrication Process:

2. A ZnS sol-gel precursor solution was made by dissolving Zinc Acetate and thiourea in a propanol/water mixture (ratio 6:1) to a concentration of 0.05 M. On the NiO thin film, a layer of ZnS was deposited via spin-coating at 3000 rpm for 10 sees, forming an intermediate layer between the HTL and the luminescent layer. This layer was 10 nm in thickness.

3. A solution consisting of colloidal CdSe/ZnS core-shell nanocrystals capped with 5- aminopentanol (to render them soluble in alcoholic solvents) dispersed in propanol was prepared. This solution was then deposited on the NiO layer via spin-coating at 3000 rpm for 10 sees. The device was then heated at 100° C for 5 minutes under nitrogen to evaporate the solvent, forming a luminescent thin film with a thickness of 35 ran.

4. On the luminescent QD thin film, a second layer of ZnS was deposited via spin- coating at 3000 rpm for 10 sees, forming an intermediate layer between the luminescent layer and the ETL. This layer was 10 nm in thickness.

5. A solution consisting of colloidal ZnO nanocrystals dispersed in ethanol (concentration 0.1 M) was prepared via a low temperature base-catalyzed reaction well described in the literature The nanocrystals were washed twice via repeated precipitation in hexane before being redispersed in fresh ethanol. The particle diameter as measured by electron microscopy was between 3-8 nm. The solution was deposited on the luminescent QD layer via spin-coating at 3000 rpm for 10 sees. The device was then heated at 100° C for 5 minutes under nitrogen, forming an ETL with a thickness of 40 nm.

6. Metallic silver was then deposited on the ZnO layer through a patterned mask to form the device cathode.

Device Testing and Performance

The graph in Figure 6 showing an absorption profile of the NiO thin film material prepared using the methods described above. The material is highly transparent over the visible region and is therefore highly suitable as a HTL in a QD-LED.

The graph in Figure 7 with the line labeled Device A showing the current-voltage behaviour of the device described in Example 1. The line labeled Device B shows the current- voltage behaviour of an alternate device which is otherwise identical to Example 1 but which has the ETL omitted. The inclusion of the ETL clearly enhances the injection of electrons into the active layer, as evidenced by the reduced turn-on voltage and improved current flow. The graph in Figure 8 shows the spectrum of the photoluminescence and electroluminescence measured from the device described in Example 1. The dashed line represents the photoluminescence spectrum, showing contributions from both the active layer and from the ZnO contained in the ETL are visible in the emission spectrum. The solid line represents the electroluminsecence spectrum, showing pure emissions from the active layer.

The graph in Figure 9 shows a plot of the luminous intensity as a function of applied voltage for the device described in Example 1. The measurements were made under ambient conditions with the light intensity measured via a calibrated silicon photodiode.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A method of manufacturing an inorganic semiconductor device including nanocrystals which comprises an inorganic electron transport layer (ETL), an inorganic semiconductor nanocrystal layer, and an inorganic hole transport layer (HTL), said method comprising the steps of forming the inorganic electron transport layer, inorganic semiconductor nanocrystal layer and inorganic hole transport layer by wet-chemical deposition processes.

2. A method of manufacturing an inorganic semiconductor device including nanocrystals which comprises an inorganic electron transport layer (ETL), an inorganic semiconductor nanocrystal layer, and an inorganic hole transport layer (HTL), said method comprising:

(b) depositing the precursor sol onto a surface; and

3. A method of manufacturing an inorganic semiconductor device including nanocrystals which comprises an inorganic electron transport layer (ETL), an inorganic semiconductor nanocrystal layer, and an inorganic hole transport layer (HTL), said method comprising:

(i) forming the inorganic HTL by wet-chemical deposition;

(iii) forming the inorganic ETL by wet-chemical deposition including the steps: (a) preparing a precursor sol by dissolving one or more metal salts into a liquid solution to a concentration of 0.01 to 1.0M;

(b) depositing the precursor sol onto a surface; and

4. A method of manufacturing an inorganic semiconductor device including nanocrystals which comprises an inorganic electron transport layer (ETL), an inorganic semiconductor nanocrystal layer, and an inorganic hole transport layer (HTL), said method comprising:

(i) forming the inorganic HTL by wet-chemical deposition;

(iii) forming the inorganic ETL by wet-chemical deposition including the steps: (a) preparing a precursor sol by dispersing one or more nanoparticles in a solution;

(b) depositing the precursor sol onto a surface; and

5. A method of manufacturing an inorganic semiconductor device including nanocrystals which comprises an inorganic electron transport layer (ETL), an inorganic semiconductor nanocrystal layer, and an inorganic hole transport layer (HTL), said method comprising: (i) forming the HTL by wet-chemical deposition including the steps of:

(a) preparing a precursor sol by dissolving one or more metal salts into a liquid solution to a concentration of 0.01 to l.OM;

(b) depositing the precursor sol onto a surface; and

6. A method of manufacturing an inorganic semiconductor device including nanocrystals which comprises an inorganic electron transport layer (ETL), an inorganic semiconductor nanocrystal layer, and an inorganic hole transport layer (HTL), said method comprising:

(b) depositing the precursor sol onto a surface; and

(a) preparing a precursor sol by dispersing one or more nanoparticles in a solution; (b) depositing the precursor sol onto a surface; and

7. A method according to any one of claims 1 to 6, including the steps of forming the semiconductor nanocrystal layer on the HTL and subsequently forming the ETL on the semiconductor nanocrystal layer.

8. A method according to any one of claims 1 to 6, including the steps of forming the semiconductor nanocrystal layer on the ETL and subsequently forming the HTL on the semiconductor nanocrystal layer.

9. A method according to any one of claims 1 to 8 wherein the HTL precursor sol is formed by dissolving one or more metal salts into a liquid solution to a concentration of between 0.3 - 0.6M.

10. A method according to any one of claims 1 to 9 wherein the inorganic HTL is prepared at a heating temperature at 400 ⁰C - 500 ⁰C.

11. A method according to any one of claims 1 to 10, wherein the HTL exhibits high transparency over the visible wavelength region and exhibits an electrical resistivity of less than 100 Ωcm.

12. A method according to any one of claims 1 to 10 wherein the ETL precursor sol is formed by dissolving one or more metal salts into a liquid solution to a concentration of between 0.3 - 0.6M.

13. A method according to any one of claims 1 to 11 wherein the inorganic ETL is prepared at a heating temperature of 50 - 100 ⁰C.

14. A method according to any one of claims 1 to 13, wherein the ETL exhibits an electrical resistivity of less than 100 Ωcm.

15. A method according to any one of claims 1 to 14, wherein the HTL is NiO based.

16. A method according to any one of claims 1 to 15, wherein the ETL is ZnO based.

17. A method according to any one of claims 2 to 6, wherein the depositing step comprises spin-coating.

18. A method according to any one of claims 1 to 17, wherein the inorganic semiconductor layer is formed by wet-chemical deposition which includes a heating step at a temperature range of 50-250°C, preferably 50-100 ⁰C.

19. A method according to any one of claims 1 to 18, wherein the HTL and ETL are deposited from a solution wherein some condensation has already taken place.

20. A method according to any one of claims 2 to 6, wherein the precursor sol is heated prior to deposition.

21. A semiconductor device prepared by a method according to any one of claims 1 to 20.

22. A semiconductor device according to claim 21 which is a QD-LED.

23. A semiconductor device according to claim 21 or claim 22 which also further comprises two electrodes and a transparent substrate layer.

24. A semiconductor device comprising an inorganic electron transport layer (ETL), an inorganic semiconductor nanocrystal layer, and an NiO based hole transport layer (HTL).

25. A semiconductor device according to claim 24 wherein the ETL is ZnO based.

26. A semiconductor device according to claim 24 or claim 25, wherein the semiconductor nanocrystal layer is formed on the ETL and the HTL is formed on the semiconductor nanocrystal layer.

27. A semiconductor device according to any one of claims 24 to 26, wherein the HTL exhibits high transparency over the visible wavelength region and exhibits an electrical resistivity of less than 100 Ωcm.

28. A semiconductor device according to any one of claims 24 to 27, wherein the ETL exhibits an electrical resistivity of less than 100 Ωcm.

29. A semiconductor device according to any one of claims 24 to 28, which further comprises two electrodes and a transparent substrate layer.

30. A semiconductor device according to any one of claims 24 to 29, which is a QD- LED.

31. A semiconductor device substantially as hereinbefore described with reference to Figures 2 to 5.