WO2014007761A1

WO2014007761A1 - Method of preparing an electrically conductive material, and an electrically conductive material

Info

Publication number: WO2014007761A1
Application number: PCT/SG2013/000276
Authority: WO
Inventors: Ghim Wei HO; Kevin Moe
Original assignee: National University Of Singapore
Priority date: 2012-07-05
Filing date: 2013-07-04
Publication date: 2014-01-09

Abstract

There is provided a method of preparing an electrically conductive material. The method comprises providing a substrate in association with a plurality of growth sites; and contacting at least part of the growth sites with a growth solution to cause a continuous layer of electrically conductive crystalline metal oxide to grow from the growth sites, wherein the growth solution comprises metal ions for growing the crystalline metal oxide. There is also provided an electrically conductive material obtainable by said method.

Description

METHOD OF PREPARING AN ELECTRICALLY CONDUCTIVE MATERIAL, AND AN ELECTRICALLY CONDUCTIVE MATERIAL

TECHNICAL FIELD

The present disclosure relates broadly to electrically conductive materials and methods of preparing electrically conductive materials. The present disclosure also relates, for example, to transparent conducting oxides.

BACKGROUND

Transparent conducting oxides (TCO) are materials which are both transparent and conducting. They are used as the front electrode of solar cells because they are able to collect photocurrent while simultaneously allowing light to pass through. Wide band gap metal-oxide semiconductors have been used as transparent conductors (TCs) since the early 20th century. Tin-doped indium oxide has been used particularly for display panels because of its high transparency and low resistivity. However, the high cost of indium has spurred the development of alternatives such as suitably doped zinc oxide and tin oxide based TCs. These TCs are deposited using vacuum deposition techniques such as sputtering, pulsed laser deposition (PLD) and chemical vapour deposition (CVD). More specifically, at an industrial level, TCOs are fabricated by sputtering or CVD. Besides metal oxides, developments include the use of metallic nanowires meshes, conducting polymers, as well as carbon allotropes, for example, carbon nanotubes (CNTs) and graphene. While these alternatives provide low-cost and scalable TCs, they have inherent drawbacks and limitations that impede commercialization. For example, polymer TCs suffer from stability issues as they easily degrade in oxidizing environments and under UV illumination, while chemically synthesized reduced graphene- oxide exhibits high sheet resistances. In addition, carbon nanotubes, graphenes, metal nanowires and others suffer from conflicting high transparency and low conductivity factors. Furthermore, high temperature multi-step processing, aggressive/toxic chemical treatment and etching of graphene and CNTs TCs have inevitably imposed higher cost fabrication and environmental concerns. Thus far, metal oxides are still by far one of the most prevalent forms of TC because of their low resistivities, excellent transparency, environmental stability and compatibility for rigid applications such as large scale PV installations, electro-chromic windows and transparent heaters. However, there has not been any satisfactory method of fabricating metal oxides TCs that is simple and low cost and which can be easily scaled to an industrial level.

In view of the above, there is a need to provide an electrically conductive material and a method of preparing said electrically conductive material that address or at least ameliorate the above drawbacks. Particularly, there is also a need to provide a metal oxide TC and a method of preparing said metal oxide TC that address or at least ameliorate the above drawbacks. SUMMARY

According to one aspect, there is provided a method of preparing an electrically conductive material, the method comprising providing a substrate in association with a plurality of growth sites; and contacting at least part of the growth sites with a growth solution to cause a continuous layer of electrically conductive crystalline metal oxide to grow from the growth sites, wherein the growth solution comprises metal ions for growing the crystalline metal oxide.

In one embodiment, the method further comprises providing the growth solution at a supersaturation temperature prior to the contacting step.

In one embodiment, the supersaturation temperature is no less than about 55 °C.

In various embodiments, the crystalline metal oxide layer is substantially transparent to light.

In various embodiments, the growth solution is provided at a volume that is dependent on an area of the continuous layer of electrically conductive crystalline metal oxide to be grown such that the ratio of the volume of growth solution provided to the area of the continuous layer of electrically conductive crystalline metal oxide to be grown is from about 15ml/cm² to about 30ml/cm², when the concentration of the growth solution is from about 25 to about 60 mM. In various embodiments, the growth sites and the crystalline metal oxide layer comprise a metal oxide selected from the group consisting of zinc oxide, tin oxide and titanium dioxide. In various embodiments, the growth solution further comprises one or more dopants.

In one embodiment, the one or more dopants comprise at least one of a rare earth metal, a metal from Group III of the periodic table of elements or both.

In various embodiments, the contacting step comprises contacting the at least part of the growth sites with the growth solution for no more than about 3 hours.

In various embodiments, the step of providing the substrate in association with the plurality of growth sites comprises forming a seed layer comprising the growth sites over the substrate. In various embodiments, the method further comprises inhibiting the growth of the crystalline metal oxide layer when it reaches a predetermined thickness.

In various embodiments, the substrate comprises at least one of an amorphous substrate, a crystalline substrate, a polycrystalline substrate or a mixture thereof.

In various embodiments, the method further comprises forming a barrier layer over the crystalline metal oxide layer; and removing part of the barrier layer to expose a region of the underlying crystalline metal oxide layer to allow further growth of the crystalline metal oxide layer in the exposed region.

In one embodiment, the method further comprises contacting the crystalline metal oxide layer with a growth solution to further grow a primary projection from the exposed region of the crystalline metal oxide layer.

In one embodiment, the method further comprises forming a seed layer comprising a plurality of growth sites over the primary projection; contacting the primary projection with a growth solution to further grow secondary projections from the primary projection.

In various embodiments, the method is carried out under non-vacuum conditions.

According to another aspect, there is provided an electrically conductive material comprising a substrate in association with growth sites; and a continuous layer of electrically conductive crystalline metal oxide grown from at least part of the growth sites.

In one embodiment, the crystalline metal oxide layer is substantially transparent to light.

In various embodiments, the crystalline metal oxide layer has a surface root mean square (rms) roughness in the range of from about 5 nm to about 12 nm.

In various embodiments, the crystalline metal oxide layer has a grain size in the range of from about 500 nm to about 3 pm. In various embodiments, the electrically conductive material further comprises a seed layer adjacent to the substrate, wherein the seed layer comprises the growth sites. In various embodiments, the crystalline metal oxide layer is co-doped with at least one of a rare earth metal, a metal from Group III of the periodic table of elements or both.

In various embodiments, the crystalline metal oxide layer comprises one or more primary projections extending from the crystalline metal oxide layer; and wherein an interface between each of the one or more primary projections and the crystalline metal oxide layer comprises a homojunction.

In various embodiments, the crystalline metal oxide layer comprises one or more secondary projections extending from the one or more primary projections.

DEFINITIONS The term "electrically conductive material" as used herein is to be interpreted broadly to include but not limited to both a conductive material, which is intrinsically or inherently capable of electrical conductivity, and a semiconductive material, which exhibits semiconducting properties. The term "substrate" as used herein is to be interpreted broadly to refer to any supporting structure.

The term "layer" when used to describe a first material is to be interpreted broadly to refer to a first depth of the first material that is distinguishable from a second depth of a second material. The first material of the layer may be present as a continuous film, as discontinuous structures or as a mixture of both. The layer may also be of a substantially uniform depth throughout or varying depths. Accordingly, when the layer is formed by individual structures, the dimensions of each of individual structure may be different. The first material and the second material may be same or different and the first depth and second depth may be same or different.

The term "continuous" when used to describe a film or a layer is to be interpreted broadly to refer to a film or a layer that is substantially without gaps or holes or voids across the film or layer. In this regard, a continuous film or a continuous layer is also intended to include a film or a layer that may have trivial gaps or holes or voids that may not appreciably affect the desired properties of the film or the layer. For example, if the layer or film is a conductive layer or conductive film, the gaps or holes or voids present (if any) may be at an atomic level such that the desired electrical properties of the layer or film is not compromised. Accordingly, it is also appreciated that the continuous layer or continuous film disclosed herein may be formed from very closely and densely packed structures. For instance, the continuous layer or continuous film may be grown from densely packed growth sites where each growth site is in proximity or abutting its adjacent growth sites. The continuous layer or continuous film may therefore also be a collection of columnized structures, each columnized structure being grown from each growth site with each columnized structure in proximity or abutting its adjacent columnized structures, thereby forming grain interface or atomic interface therebetween. As mentioned above, in some examples where the layer or film is a conductive layer or conductive film, the grain interface or the atomic interface between the structures does not undesirably compromise the electrical conductivity of the continuous layer or continuous film. In some embodiments, where the columnized structures have crystalline structures, the columnized structures may have the same crystal orientation. Thus, in certain cases, the continuous layer or continuous film also covers a continuous crystalline layer or a continuous crystalline film without amorphicity in between its structures. Therefore in some instances, a crystalline continuous layer or a crystalline continuous film may interchangeably be referred to as a textured single crystal layer or a textured single crystal film.

The term "grow" when used herein is to be broadly interpreted to mean increasing the mass of a material. In some examples, when the term "grow" is used with respect to a crystalline structure, it includes an addition of new atoms or ions into or as a part of a crystalline lattice.

The term "supersaturation" as used herein is to be interpreted broadly to mean the situation where the solution concentration exceeds the equilibrium solubility of its solute. The term "supersaturation temperature" is thus to be interpreted to mean the temperature at which this situation occurs.

The term "substantially transparent to light" when used herein to describe an object is to be interpreted broadly to mean that 50% or more of the incident light normal to surface of the object can be transmitted through the object. In some examples, the object that is substantially transparent to light allow 60% or more, 65% or more, 70% or more, 80% or more, 85% or more, 90% or more or 95% or more of the incident light normal to surface of the object to be transmitted. In one example, the object that is substantially transparent to light allow above 70% of the incident light normal to surface of the object to be transmitted. The term "dopant" as used herein is to be interpreted broadly to include any material, element or atom that is incorporated or is to be incorporated in trace amounts in a base material.

The term "micro" as used herein is to be interpreted broadly to include dimensions from about 1 micron to about 000 microns.

The term "nano" as used herein is to be interpreted broadly to include dimensions less than about 1000 nm.

The term "particle" as used herein broadly refers to a discrete entity or a discrete body. The particle described herein can include an organic, an inorganic or a biological particle. The particle used described herein may also be a macro-particle that is formed by an aggregate of a plurality of sub-particles or a fragment of a small object. The particle of the present disclosure may be spherical, substantially spherical, or non-spherical, such as irregularly shaped particles or ellipsoidally shaped particles. The term "size" when used to refer to the particle broadly refers to the largest dimension of the particle. For example, when the particle is substantially spherical, the term "size" can refer to the diameter of the particle; or when the particle is substantially non-spherical, the term "size" can refer to the largest length of the particle.

The terms "coupled" or "connected" as used in this description are intended to cover both directly connected or connected through one or more intermediate means, unless otherwise stated.

The term "associated with", used herein when referring to two elements refers to a broad relationship between the two elements. The relationship includes, but is not limited to a physical, a chemical or a biological relationship. For example, when element A is associated with element B, elements A and B may be directly or indirectly attached to each other or element A may contain element B or vice versa. The term "adjacent" used herein when referring to two elements refers to one element being in close proximity to another element and may be but is not limited to the elements contacting each other or may further include the elements being separated by one or more further elements disposed therebetween.

The term "and/or", e.g., "X and/or Y" is understood to mean either "X and Y" or "X or Y" and should be taken to provide explicit support for both meanings or for either meaning. Further, in the description herein, the word "substantially" whenever used is understood to include, but not restricted to, "entirely" or "completely" and the like. In addition, terms such as "comprising", "comprise", and the like whenever used, are intended to be non-restricting descriptive language in that they broadly include elements/components recited after such terms, in addition to other components not explicitly recited. Further, terms such as "about", "approximately" and the like whenever used, typically means a reasonable variation, for example a variation of +/- 5% of the disclosed value, or a variance of 4% of the disclosed value, or a variance of 3% of the disclosed value, a variance of 2% of the disclosed value or a variance of 1% of the disclosed value.

Furthermore, in the description herein, certain values may be disclosed in a range. The values showing the end points of a range are intended to illustrate a preferred range. Whenever a range has been described, it is intended that the range covers and teaches all possible sub-ranges as well as individual numerical values within that range. That is, the end points of a range should not be interpreted as inflexible limitations. For example, a description of a range of 1% to 5% is intended to have specifically disclosed sub-ranges 1% to 2%, 1 % to 3%, 1 % to 4%, 2% to 3% etc., as well as individually, values within that range such as 1 %, 2%, 3%, 4% and 5%. The intention of the above specific disclosure is applicable to any depth/breadth of a range.

Additionally, when describing some embodiments, the disclosure may have disclosed a method and/or process as a particular sequence of steps. However, unless otherwise required, it will be appreciated that the method or process should not be limited to the particular sequence of steps disclosed. Other sequences of steps may be possible. The particular order of the steps disclosed herein should not be construed as undue limitations. Unless otherwise required, a method and/or process disclosed herein should not be limited to the steps being carried out in the order written. The sequence of steps may be varied and still remain within the scope of the disclosure.

DESCRIPTION OF EMBODIMENTS

Exemplary, non-limiting embodiments of a method of preparing an electrically conductive material and an electrically conductive material are disclosed hereinafter. The method of preparing an electrically conductive material may comprise providing a substrate in association with a plurality of growth sites; and contacting at least part of the growth sites with a growth solution to cause a continuous layer of electrically conductive crystalline metal oxide to grow from the growth sites, wherein the growth solution comprises metal ions for growing the crystalline metal oxide. The metal ions of the growth solution may comprise ions of the same metal of the metal component of the crystalline metal oxide. The growth sites may be present in a discrete layer separate from the substrate and therefore the substrate in association with the plurality of growth sites may include the situation where the substrate is in contact with the discrete layer of growth sites. In some embodiments, the growth sites are in the form of seed particles. The substrate in association with the plurality of growth sites may also include the situation where the substrate contains the growth sites such that they are present within the substrate, for example, the substrate may be doped with seed particles. In this situation, the seed particles may be at least partially exposed for contact with the growth solution.

In one embodiment, the growth sites comprise metal oxide particles. The growth sites may comprise or may be zinc oxide particles, or titanium dioxide (or titania) particles, or tin oxide particles or mixtures thereof. In one embodiment, the growth sites comprise zinc oxide particles. Advantageously, ZnO is abundant in nature, is substantially low in cost and has non-toxic nature. In some examples, the term "growth site" may be interchangably referred to as a nucleation site. In some embodiments, the growth sites exclude indium or doped/undoped indium oxide growth sites (for example, indium or doped/undoped indium oxide seed particles) or do not comprise indium or doped/undoped indium oxide growth sites. In some embodiments, the growth sites may comprise or may be undoped zinc oxides particles. The growth sites may be micro-sized particles or nano-sized particles. In some embodiments, the sizes of the growth sites, for example, seed particles are in the range of from about 5 nm to about 20 nm.

In one embodiment, the electrically conductive material comprises a semiconductor material. Advantageously, the electrically conductive material may be suitable for use in a photovoltaic cell or in sensors. The continuous layer of electrically conductive crystalline metal oxide may be a substantially planar layer. In some embodiments, the continuous layer of electrically conductive crystalline metal oxide comprises one or more surface structures for example, projections, elongations, protrusions, extensions, pillars, trenches, throughs, cavities or depressions. In one embodiment, the crystalline metal oxide layer is substantially transparent to light. In one embodiment, the continuous layer of electrically conductive crystalline metal oxide allows at least 70% of the incident light to be transmitted through it. Advantageously, this allows the continuous layer of electrically conductive crystalline metal oxide to be used effectively in photovoltaic or solar applications. The continuous layer of electrically conductive crystalline metal oxide may comprise zinc oxide or titanium dioxide (or titania) particles, or tin oxide particles or mixtures thereof.

The growth solution may further comprise at least one of nitrate, sulphate ions, acetate ions, chloride ions, oxide, hydroxide or mixtures thereof. For example, for causing a continuous layer of crystalline zinc oxide to grow, the growth solution may comprise at least one of zinc sulphate, zinc acetate, zinc chloride, zinc nitrate, zinc oxide, zinc hydroxide or mixtures thereof. In one embodiment, the growth solution further comprises a base. The base comprises ammonia (NH₃). It will be appreciated that other suitable bases may also be used. In other examples, the base or ligand may be at least one of ammonia, sodium hydroxide, potassium hydroxide, ammonium nitrate, ammonium acetate, ammonium chloride, ammonium sulphate, hexamethyltetraamine (HMT) or mixtures thereof. In one embodiment, the growth solution is alkaline and the pH of the growth solution is more than about 7, more than about 8, more than about 9, more than about 9.5, more than about 10, more than about 10.5. In one embodiment, the pH of the growth solution is about 10.85. The growth solution may also comprise a surfactant. In one embodiment, the surfactant comprises trisodium citrate. It will be appreciated that other suitable surfactants may also be used. In other examples, the surfactant may be at least one of trisodium citrate, tripotassium citrate, ammonium citrate, citric acid, gallium citrate, aluminum citrate, indium citrate, zinc citrate, triethylcitrate or combinations thereof. Advantageously, the presence of a surfactant may promote the growth of the continuous layer of crystalline metal oxide layer. The growth solution may also comprise one or more dopants such that the crystalline metal oxide layer formed will be doped by the one or more dopants. The one or more dopants comprise at least one of a rare earth metal or a metal from Group III of the periodic table of elements. In one embodiment, the metal from Group III of the periodic table of elements comprises a metal from Group IIIA of the periodic table of elements. The rare earth metal may be selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and combinations thereof. In one embodiment, the metal from Group III of the periodic table of elements comprises gallium. In one example, the metal from Group III of the periodic table of elements may include aluminium. In one embodiment, the dopant comprises at least one of yttrium or gallium. It will be appreciated that other suitable dopants may also be used. In some examples, the dopant may also comprise a metal that is capable of being solubilised as metal ions in the growth solution. The at least one of the rare earth metal or the metal from Group III of the periodic table of elements may be provided as a nitrate solution, nitrate hexahydrate solution or mixtures thereof.

In one embodiment, the method further comprises providing the growth solution at a supersatu ration temperature prior to the contacting step. The supersaturation temperature may be no less than about 55 °C, no less than about 60 °C, no less than about 65 °C, no less than about 70 °C, no less than about 75 °C or no less than about 80 °C . The step of providing the solution at the supersaturation temperature may comprise increasing or decreasing the temperature of the solution to the supersaturation temperature. In one embodiment, the growth solution is provided at a volume that is dependent on an area of the continuous layer of electrically conductive crystalline metal oxide to be grown such that the ratio of the volume of growth solution provided to the area of the continuous layer of electrically conductive crystalline metal oxide to be grown is from about 0ml/cm² to about 50ml/cm², from about 15ml/cm² to about 45ml/cm², from about 20ml/cm² to about 35ml/cm², or from about 25ml/cm² to about 30ml/cm². The aforesaid ratio may depend on the concentration of the growth solution. For example, the concentration of the growth solution may be from about 25 mM to about 60 mM, from about 30 mM to about 55 mM, from about 35 mM to 50 mM, from about 40 mM to about 45 mM. The area of the continuous layer of electrically conductive crystalline metal oxide to be grown may be the average planar area surface area of the continuous layer of electrically conductive crystalline metal oxide to be grown.

In one embodiment, the growth sites and the crystalline metal oxide layer comprise zinc oxide. Advantageously, ZnO is abundant in nature, is substantially low in cost and has non-toxic nature. The zinc oxide may comprise dopants such as those discussed above. In some embodiments, the crystalline metal oxide layer excludes indium, or doped/undoped indium oxide or does not comprise indium or doped/undoped indium oxide. In some embodiments, the growth sites comprise non-doped zinc oxide and the crystalline metal oxide layer comprises co-doped YGa-ZnO. In one embodiment, the step of contacting at least part of the growth sites with a growth solution to cause a continuous layer of electrically conductive crystalline metal oxide to grow from the growth sites is carried out in one single step. Advantageously, in such embodiment, the contact of the growth sites with the growth solution does not need to be removed and then re- established in a separate step to allow a continuous layer of electrically conductive crystalline metal oxide to form. More advantageously, this shortens processing time, increases processing efficiency and avoids the need for additional steps (which may bring about additional cost and efforts) to establish a continuous layer of electrically conductive crystalline metal oxide to be formed. In one embodiment, the contacting step comprises contacting at least part of the growth sites with the growth solution for no more than about 4 hours, no more than about 3.5 hours, no more than about 3 hours, no more than about 2.5 hours or no more than about 2 hours. Advantageously, but controlling the contacting time such that it is not excessive, undesired dissolution of the growth sites may be avoided. The step of contacting at least part of the growth sites with the growth solution may comprise applying a magnetic field to bring the at least part of the growth sites in contact with the growth solution. In some embodiments, contacting the at least part of growth sites with the growth solution includes immersing at least part of the substrate in association with the growth sites into the solution. The magnetic field may be provided by a pair of magnets arranged to attract to each other to control at least one of a movement or an orientation of the substrate, wherein one of the pair of magnets is connected to the substrate and the other of the pair of the magnets is connected to a movement controller (for example, a substrate manipulator) outside a vessel containing the solution. When in contact with the growth solution, the substrate may also be rotated to allow a substantially uniform continuous layer of crystalline metal oxide to grow. The rotation may also be carried out utilizing a magnetic field and/or the pair of magnets described above.

In one embodiment, the step of providing a substrate in association with the plurality of growth sites comprises forming a seed layer comprising the growth sites over the substrate. Accordingly, the seed layer may be formed adjacent to the substrate. Therefore, in some embodiments, the seed layer is ultimately sandwiched between the substrate and the crystalline metal oxide layer that is subsequently formed. The step of forming the seed layer may comprise depositing the seed layer over the substrate. Physical and/or chemical deposition techniques to deposit the seed layer may be used. For example, physical and/or chemical deposition techniques may include pulsed laser deposition, sputtering, atomic layer deposition, chemical vapor deposition, sol- gel, dip coating, drop casting, electrodeposition, spray pyrolysis; and successive ionic layer adsorption and reaction (SILAR). In one embodiment, the step of forming a seed layer comprising the growth sites over the substrate comprises spin-coating a colloidal dispersion comprising the growth sites onto the substrate. Prior to the step of depositing the seed layer over the substrate, the method may also comprise the cleaning the substrate with at least one of acetone, isopropyl alcohol, or isopropyl ethanol in an ultrasonic bath; and drying the substrate. It will be appreciated that other suitable cleaning solutions may also be used to clean the substrate.

In one embodiment, the method disclosed herein comprises inhibiting the growth of the crystalline metal oxide layer when it reaches a predetermined thickness. This may be carried out by removing contact of the growth solution with the crystalline metal oxide layer when it reaches the predetermined thickness. In some embodiments, when the substrate in association with the growth sites is immersed in the growth solution, the step of removing contact of the growth solution with the crystalline metal oxide layer comprise removing the substrate from the growth solution. The step of removing contact of the growth solution with the crystalline metal oxide layer may also comprise washing the substrate and the crystalline metal oxide layer with deionized water after the substrate is removed from the growth solution. The step of removing contact of the growth solution with the crystalline metal oxide layer may also comprise the application of a magnetic field and/or the use of a pair of magnets as described herein. The method disclosed herein may further comprise in-situ measuring the thickness of the crystalline metal oxide layer to determine whether the predetermined thickness has been reached. The step of in-situ measuring the thickness of the crystalline metal oxide layer may comprise exposing a near- ultraviolet light to a side of the substrate while being exposed in the solution; detecting the intensity of the light at an opposing side of the substrate; and determining the thickness of the crystalline metal oxide layer based on the detected intensity of the light. In one embodiment, the predetermined thickness of the substrate is in the range of from about 6.0 micron to about 6.5 micron. It will also be appreciated that a transmittance based method, reflectance based method or a combination thereof may be used to in-situ determine the thickness of the crystalline metal oxide layer.

In one embodiment, the method disclosed herein further comprises forming a barrier layer over the crystalline metal oxide layer; and removing part of the barrier layer to expose a region of the underlying crystalline metal oxide layer to allow further growth of the crystalline metal oxide layer in the exposed region. The barrier layer may comprise a polymer. In one embodiment, the barrier layer comprises Poly(methyl methacrylate) (PMMA). It will be appreciated that any other barrier layer that can substantially inhibit further growth of the crystalline metal oxide layer may be used. The barrier layer may be a positive or negative photoresist layer. The step of removing part of the barrier layer to expose a region of the underlying crystalline metal oxide layer may comprise using etching and/or using lithography to selectively remove part of the barrier layer. This may be carried out by the further use of patterned masks. In one embodiment, the step of removing part of the barrier layer to expose a region of the underlying crystalline metal oxide layer comprises adding an organic solvent to the part of the barrier layer to be removed. The organic solvent may be, for example, acetone. Other suitable organic solvents that are capable of removing the part of the barrier through dissolution or otherwise may also be used.

In this regard, in one embodiment, the method further comprises contacting the crystalline metal oxide layer with a growth solution to further grow a primary projection from the exposed region of the crystalline metal oxide layer. In this step, the growth solution may be same or may have the same contents as the growth solution used to contact the growth sites. Alternatively, the growth solution may be different from the growth solution used to contact the growth sites; for example, the growth solution used to further grow a primary projection from the exposed region of the crystalline metal oxide layer may comprise one or more dopants that are different from the growth solution used to contact the growth sites. Thus, the primary projection may have a different chemical composition from the crystalline metal oxide layer grown from the growth sites. In some embodiments, when the primary projection has a different chemical composition from the crystalline metal oxide layer grown from the growth sites, the energy band gap of the primary projection is substantially the same as the energy band of the crystalline metal oxide layer grown from the growth sites. Accordingly, in some embodiments, the interface between the primary projection and the crystalline metal oxide layer comprises a homojunction. In various embodiments, the homojunction provides a barrier- less interface with minimal interface discontinuity that facilitates the movement of electrons. The lattice structure and/or crystalline orientation of the primary projection may be substantially the same as the lattice structure and/or crystalline orientation of the crystalline metal oxide layer grown from the growth sites. In other embodiments, the primary projection has substantially the same chemical composition as the crystalline metal oxide layer grown from the growth sites. The steps of forming a barrier layer, exposing a region of the underlying material to allow further growth and contacting the exposed material to the growth solution may also be repeated on the primary projection to allow further projections to grow therefrom. Likewise, these steps may also be repeated on these further projections as desired to form more projections. Accordingly, the method disclosed herein may be used to form branched crystalline metal oxide structures.

The method disclosed herein may also comprise forming a seed layer comprising a plurality of growth sites over the primary projection; and contacting the primary projection with a growth solution to further grow secondary projections from the primary projection. In this step, the growth solution may be same or may have the same content as the growth solution used to contact the crystalline metal oxide layer or the growth solution used to contact the growth sites. Alternatively, the growth solution may be different from the growth solution used to contact the crystalline metal oxide layer or the growth solution used to contact the growth sites; for example, the growth solution used to further grow secondary projections from the exposed region of the crystalline metal oxide layer may comprise one or more dopants that are different from the growth solution used to contact the crystalline metal oxide layer or the growth solution used to contact the growth sites. Thus, the secondary projections may have a different chemical composition from the primary projection or the crystalline metal oxide layer grown from the growth sites. In some embodiments, when the secondary projections have a different chemical composition from the primary projection, the energy band gap of the secondary projections is substantially the same as the energy band of the primary projection. Accordingly, in some embodiments, the interface between each of the secondary projection and the primary projection comprises a homojunction. The lattice structure and/or crystalline orientation of the secondary projections may be substantially the same as the lattice structure and/or crystalline orientation of the primary projection. In other embodiments, the secondary projections have substantially the same chemical composition as the primary projection. The seed layer comprising a plurality of growth sites formed over the primary projection may be different or the same as the seed layer formed over the substrate. Accordingly, the growth sites formed over the primary projection may be different or the same as the growth sites in association with the substrate.

The substrate disclosed herein may be any substrate that is capable of supporting the growth of the crystalline metal oxide layer. The substrate may be amorphous, crystalline, polycrystalline or mixtures thereof. In one embodiment, the substrate is amorphous. Advantageously, the method disclosed herein is capable of allowing the continuous layer of crystalline metal oxide to be secured on an amorphous substrate even though there may be a lattice mismatch. The substrate may be a non-conductor, a conductor or a semiconductor. In one embodiment, the substrate is selected from a group consisting of glass (S1O₂), polymer, silicon and a metal oxide layer. In one embodiment, the substrate is a flexible polymeric substrate. In various embodiments, the lattice of the substrate and the lattice of the continuous crystalline metal oxide layer are mismatched or non-matching. Advantageously, the method disclosed herein is capable of preparing an electrically conductive material on a flexible polymeric substrate, which is generally not possible via conventional deposition techniques. As discussed herein, in some embodiments, the electrically conductive material may comprise a continuous layer of crystalline metal oxide with primary projection and/or secondary projections. Thus, in some embodiments, the substrate described herein may also be construed to comprise at least one of a continuous layer of crystalline metal oxide, a continuous layer of crystalline metal oxide with primary projection and/or secondary projections, or a continuous layer of crystalline metal oxide with further projections or combinations thereof. The substrate may be opaque, translucent or substantially transparent to light. Advantageously, when the substrate is substantially transparent to light, in-situ transmission-based optical measurement of the thickness of the crystalline metal oxide layer may be facilitated.

Some of the steps of the disclosed method may be carried out under hydrothermal conditions and thus the method may also comprise a hydrothermal synthesis process. The method disclosed herein may be carried out under non-vacuum conditions. The method disclosed herein may also be carried out under non-clean room environment. In one embodiment, the method disclosed herein is carried out under ambient conditions. In one embodiment, the method disclosed herein is carried out in an environment containing more than about 12 particles, more than about 15 particles, more than about 20 particles, more than about 50 particles, more than about 100 particles, more than about 500 particles, more than about 1000 particles, more than about 5000 particles, more than about 10000, more than about 50000 particles, more than about 100000 particles or more than about 500000 particles of about 1 μιη and smaller size, about 0.5 pm and smaller size or about 0.3 pm and smaller size per cubic meter. In some embodiments, the method disclosed herein excludes or does not include deposition by way of vapour having precursor materials suitable for forming the crystalline structure, such as by a chemical vapour deposition process or by a sputtering process for forming a crystalline structure. In one embodiment, the method disclosed herein excludes sputtering or does not comprise sputtering. In one embodiment, the method disclosed herein excludes chemical vapour deposition or does not comprise chemical vapour deposition.

The method disclosed herein may be carried out on an industrial scale. Therefore, there is also provided an industrial method of preparing an electrically conductive material. The electrically conductive material disclosed herein may comprise a substrate in association with growth sites; and a continuous layer of electrically conductive crystalline metal oxide grown from at least part of the growth sites. The substrate, growth sites and electrically conductive crystalline metal oxide layer may have one or more of the properties or features discussed above. The substrate may be at least one of an amorphous substrate, a crystalline substrate, a polycrystalline substrate or a mixture thereof.

In one embodiment, the crystalline metal oxide layer is substantially transparent to light. In some embodiments, the continuous layer or film is substantially transparent to light. Advantageously, the electrically conductive material can be used in applications that rely on the transmittance of light, for example in solar cell applications. In some embodiments, the crystalline metal oxide layer has a surface root mean square (rms) roughness in the range of from about 3 nm to about 20 nm, from about 4 nm to about 15 nm, from about 5 nm to about 14 nm, from about 5 nm to about 13 nm, from about 5 nm to about 12 nm. Accordingly, it will be appreciated that in some embodiments, the crystalline metal oxide layer is considered to be smooth. Advantageously, a smooth surface is essential for producing highly transparent conductors as an excessive amount of surface irregularities may cause haze. Haze may be computed by taking the ratio between the diffused and total transmission for the various structures. In some embodiments, the crystalline metal oxide layer comprises a fully coalesced surface. The crystalline metal oxide layer may have a grain size in the range of from about 500 nm to about 5 pm, about 500 nm to about 1 pm, about 1 pm to about 8 pm, about 2 pm to about 7 pm, about 3 pm to about 6 pm, about 1 pm to about 5 pm, about 1 pm to about 4 pm, or about 1 pm to about 3 pm. In one embodiment, the crystalline metal oxide layer has a grain size that is at least 3 times, at least 5 times, at least 7 times or at least 10 times larger than a metal oxide layer deposited through a deposition method. Depending on the grain size, the density of crystalline metal oxide layer may be about 4 grain/pm². Advantageously, the crystalline metal oxide layer may have substantially less defects than a metal oxide layer deposited through a conventional deposition method.

The electrically conductive material disclosed herein may further comprise a seed layer adjacent to the substrate, wherein the seed layer comprises the growth sites. Accordingly, the seed layer may be sandwiched in between the substrate and the crystalline metal oxide layer. The seed layer of the electrically conductive material may have one or more of the properties or features previously discussed above.

In one embodiment, the crystalline metal oxide layer of the electrically conductive material comprises one or more primary projections extending from the crystalline metal oxide layer, and wherein an interface between each of the one or more primary projections and the crystalline metal oxide layer comprises a homojunction. Advantageously, the homojunction may provide a barrier-less interface with minimal interface discontinuity that facilitates the movement of electrons. As high recombination occurs where there is a build-up of charges especially at poor material interfaces, it may be essential to have continuous homojunction conducting electrodes to avoid interface discontinuity which would result in fast recombination. One or more secondary projections may also extend from the one or more primary projections. Likewise, the interface between each of the one or more primary projections and each of the secondary projection may comprise a homojunction.

It will be appreciated that in some embodiments, the methods disclosed herein may be carried out in a reactor. Accordingly, there is also provided a reactor that is suitable for performing the method disclosed herein. The reactor may comprise a vessel configured to contain a solution; and a manipulator configured to expose at least part of growth sites comprised in a substrate to the solution to cause a continuous layer of electrically conductive crystalline metal oxide to grow from the growth sites, wherein the solution comprises metal ions for growing the continuous layer of crystalline metal oxide.

The reactor may further comprise a temperature controller configured to provide the solution at a supersaturation temperature, wherein the solution comprises the metal ions for growing the electrically conductive crystalline metal oxide layer at the supersaturation temperature. The manipulator may be configured to expose the at least part of the growth sites to the solution for a predetermined amount of time. In some embodiments, the manipulator is configured to immerse the substrate comprising the at least part of the growth sites into the solution using magnetic forces, for example, by applying a magnetic field. Thus, the manipulator may comprise a pair of magnets arranged to attract to each other to control at least one of a movement or an orientation of the substrate. One of the pair of magnets may be connected to the substrate and the other of the pair of the magnets may be connected to a movement controller outside the vessel. The manipulator may also be configured to remove the at least part of the growth sites from the solution after the crystalline metal oxide layer reaches a predetermined thickness. The manipulator may be configured to be operated by an operator manually or automatically.

In one embodiment, the reactor further comprises a measuring device configured to in-situ measure the thickness of the crystalline metal oxide layer to determine whether the predetermined thickness has been reached. The measuring device may be positioned external to the vessel. The measuring device may comprise a light source configured to expose a near-ultraviolet light to a side of the substrate while being exposed in the solution; a detector configured to detect the intensity of the light at an opposing side of the substrate; and a processor configured to determine the thickness of the crystalline metal oxide layer based on the detected intensity of the light.

BRIEF DESCRIPTION OF FIGURES

Fig. 1 is a flow diagram illustrating a method of preparing an electrically conductive material, in accordance with various embodiments.

Fig. 2A is a schematic diagram of an exemplary growth system illustrating a substrate that is not in contact with a growth solution, in accordance with various embodiments.

Fig. 2B is a schematic diagram of Fig. 2A illustrating the substrate being immersed in the growth solution, in accordance with various embodiments. -

Fig. 3 is a schematic diagram of Fig. 2B with an in-situ measurement system, in accordance with various embodiments.

Fig. 4A is a cross-sectional view of a scanning electron microscope (SEM) image of a film (or layer) that has been grown in a substrate, in accordance with various embodiments.

Fig. 4B is an expanded view of Fig. 4A at a higher resolution (scale bar of 500 nm), in accordance with various embodiments. Fig. 4C is a cross-sectional view of a High Resolution Transmission Electron Microscopy (HR-TEM) image of a crystal column, in accordance with various embodiments. Fig. 4D is a graph illustrating the transmission and haze measurements on an unpatterned film, trenches, and exemplary pillars, in accordance with various embodiments.

Fig. 5 is a flow chart illustrating the process flow of the preparation of an electrically conductive material, in accordance with various embodiments.

Fig. 6A is a cross-sectional (perspective) view of a scanning electron microscope (SEM) image of a film (or continuous layer) that has been grown in a substrate, in accordance with various embodiments.

Fig. 6B is an expanded view of an encircled area 6000 of Fig. 6A, in accordance with various embodiments.

Fig. 6C is a cross-sectional view of a scanning electron microscope (SEM) image of a film (or layer) that has been grown in a substrate with an extended electrode and branches, in accordance with various embodiments.

Fig. 6D is an expanded view of an encircled area 6002 of Fig. 6C, in accordance with various embodiments.

Fig. 7 is a block diagram illustrating an electrically conductive material, in accordance with various embodiments.

Fig. 8 is a schematic diagram of an exemplary electrically conductive material, in accordance with various embodiments. DETAILED DESCRIPTION OF FIGURES

Example embodiments of the disclosure will be better understood and readily apparent to one of ordinary skill in the art from the following discussions and if applicable, in conjunction with the figures. It should be appreciated that other modifications related to structural, electrical and optical changes may be made without deviating from the scope of the invention. Example embodiments are not necessarily mutually exclusive as some may be combined with one or more embodiments to form new exemplary embodiments.

Fig. 1 shows a flow diagram illustrating a method of preparing an electrically conductive material 100 in accordance with various embodiments disclosed herein. In Fig. 1 , at 102, a substrate in association with a plurality of growth sites is provided. At 104, at least part of the growth sites is contacted with a growth solution to cause a continuous layer of electrically conductive crystalline metal oxide to grow from the growth sites. The growth solution includes metal ions for growing the crystalline metal oxide. In other words, in example embodiments, the method of preparing an electrically conductive material 100 may include having a substrate with growth sites and directly contacting these growth sites with a growth solution such that a continuous layer of electrically conductive crystalline metal oxide substantially without gaps or holes throughout the layer may be prepared. This may be achieved with the continuous layer being grown from densely packed growth sites where each growth site is in proximity or abutting its adjacent growth sites. The continuous layer may be a collection of columnized structures, each columnized structure being grown from each growth site. The size or diameter of each columnized structure may be but is not limited to about 500 nm. Each columnized structure may be in proximity or abutting its adjacent columnized structures; thereby forming grain interface or atomic interface therebetween where the electrical conductivity of the continuous layer is not compromised. That is to say, the disruptions of the grain interface(s) or atomic interface(s) may be considered negligible in affecting the electrical conductivity of the continuous layer. The continuous layer of crystalline metal oxide may provide high electrical conductivity and high level of transparency. It should be appreciated that the method 100 is different from a hydrothermal synthesis method where nanorods or nanowires are relatively sparsely grown (providing gaps or holes between the nanorods), and subsequently further growth is carried out to fill those gaps and holes to form a layer.

In various embodiments, the method of preparing an electrically conductive material 100 may include a method for synthesizing or preparing a textured single crystal solution processable film for transparent conducting oxides (TCO) applications. For example, the method 100 may include a method of preparing textured single crystal Yttrium-Gallium Zinc oxide (YGa-ZnO) transparent conducting oxides (TCO) integrated with extended homojunction 3D electrodes. The TCO film may be singly or co-doped of Yttrium-Gallium-ZnO. The single crystal film growth may be on large mismatch and amorphous substrates by heteroepitaxy crystal growth. The method for preparing heteroepitaxial growth of textured single crystal singly or Co-doped -Ga-ZnO on large mismatch and amorphous substrates may not require vacuum or clean room facilities. The YGa-ZnO single crystal film may be formed/ prepared or deposited using hydrothermal synthesis (or may be interchangeably referred to as hydrothermal method).

An exemplary method (synthesis process) of enhanced optical and electrical properties of TCOs by solution processable techniques is presented as follows. In an example, rare-earth Yttrium and group III gallium doped ZnO film were synthesized on substrate by a hydrothermal method. A. CLEANING

Cleaning of the substrates was carried out using acetone, isopropyl alcohol/ethanol in an ultrasonic bath for about 10 min each before drying with compressed nitrogen (N₂) gas.

S. SEEDING B-i) The seeding solution including or consisting of a colloidal suspension of ZnO nanoparticles (which may be herein referred to as colloidal ZnO nanoparticles) was prepared by mixing 0.41 mM Zinc Acetate dihydrate (Sigma-Aldrich, US) and 0. mM Potassium Hydroxide (Sigma-Aldrich, UK) in methanol at about 60 °C for about 2 hours.

B-ii) After about 2 hours, the cloudy suspension was washed and centrifuged in methanol to give clear colloidal ZnO which was stable for about a month. B-iii) The colloidal ZnO was then spin coated several times (for example but not limited to 3 to 5 times) onto the cleaned glass substrate and dried.

For example, the step of providing a substrate in association with a plurality of growth sites at 102 of Fig. 1 may refer to the abovementioned seeding step. C. PREPARATION OF GROWTH SOLUTION

C-i) The growth solution was prepared using 35mM of zinc nitrate, Zn(N0₃)2 (Sigma-Aldrich, US) with sufficient aqueous ammonia, NH3 added to adjust the pH to 10.85.

C-ii) 1mM of sodium tricitrate was added to the abovementioned growth solution as a surfactant to promote film growth. C-iii) 0.5-3mM of gallium nitrate hexahydrate (Sigma-Aldrich, US) and

0.5-3 mM of Yttrium Nitrate (Sigma-Aldrich, US) were then added to the abovementioned growth solution as dopants.

D. PREPARATION OF GROWTH SYSTEM

To grow the film, the seeded substrate (for example, as prepared in the abovementioned seeding step (B)) was immersed in the growth solution (for example, as prepared in the abovementioned step C), and the solution was heated to its supersaturation temperature (about 65 °C) for the films to grow. However, it should be appreciated that the process may not be as trivial as it appears. Dissolution of the seed layer in the growth solution may occur if the growth solution took too long (for example, more than 15 mins or more than 30 mins) to reach its supersaturation temperature. The dissolution of the seed layer may cause the films to be rough, hazy and less conductive. Therefore, the following steps and system (or reactor 200), with reference to Figs. 2A and 2B, describe the substrate being immersed into the solution only when the temperature is sufficiently high (for example, more than 55 °C) for growth to occur while avoiding the dissolution of the seed layer. D-i) A teflon container 202, having a removable lid assembly 204 and a pair of borosilicate windows 206 along opposing walls of the container 202, was first filled with the growth solution 208. The volume of solution 208 was determined by the area of the film to be grown (~25ml/cm²).

D-ii) The seeded substrate 210 was mounted on a teflon encapsulated magnet 212a using silicone adhesive as shown in Fig. 2A.

D-iii) The seeded substrate 210 and the encapsulated magnet 212a were then secured to the inside of the removable lid assembly 204 (which was Teflon coated) using an external magnet 212b as shown in Fig. 2A. The external magnet 212b was attached to a substrate manipulator 214 which was used to position/rotate the substrate 210 within the growth system 200 without exposing the solution 208 to the surroundings while being heated. This would ensure that ammonia gas does not escape during the heating process. Use of the magnets 212a, 212b allows remote control of the substrate 210's movement and orientation. Other possible remote control means may include but is not limited to a mechanical slider. The substrate 210 was positioned above the solution waterline to at least minimize or prevent dissolution of the seed layer while the growth solution 208 heats up. The removable lid assembly 204 was shaped to accommodate the substrate 210 while maintaining a minimal volume of air above the growth solution 208. This prevents too much ammonia from escaping the solution. For example, the removable lid assembly 204 included a substantially rectangular recess having a length such that the substrate 2 0 can be received therein without being in contact with the solution.

D-iv) Heat (as shown by directional arrows 2000, 2002) was then applied to the bottom of the container 202 by means of heating tape or a water bath. This configuration created a temperature gradient across the solution 208, causing hot saturated growth solution 208 to rise to the top (of the container 202), and cold unsaturated solution 208 to sink to the bottom in a continuous convection cycle.

D-v) When a thermometer 216 indicated about 65 °C, the substrate 210 was lowered into the growth solution 208 (indicated by a directional arrow 2004 of Fig. 2A), as shown in Fig. 2B. While immersed, the substrate 210 was slowly rotated (as indicated by a directional arrow 2008) to allow and ensure the growth of an even film. For example, the speed of rotation of the substrate 210 may be about 5 rotations per min.

D-vi) The solution-substrate system 200 was left to grow for about 2 hours. Thereafter the substrate 210 was removed by raising the substrate 210 from the solution 208 (as indicated by a directional arrow 2006) and rinsed with deionized (Dl)-water.

The steps of heating the growth solution (for example, the growth solution 208) to a predetermined temperature value and then immersing the substrate (for example, the substrate 210) thereafter can be crucial to the formation of high quality films on a large scale.

E. IN-SITU MEASUREMENT OF FILM GROWTH ln-situ observation of film growth allows for real time monitoring of film thickness. The transparent nature of the films and substrate allows for optical transmission to be used to determine the thickness of the film. Fig. 3 shows an example of the in-situ measurement system 300. A low intensity near-UV light from a 385nm LED 302 was focused using a lens 304 and shone through the film (i.e., the substrate 210 of Figs. 2A and 2B) via the pair of borosilicate windows 206. The intensity of UV light was measured using a standard spectrometer 306 with a fiber optic cable 308 and a fiber optic assembly 310. The amount of light that passed through the film 210 was determined by the thickness of the film 210. Using the relationship between transmission and film thickness, the thickness of the film 210 may be determined using the following Equation 1 :-

L(t) = ~(α)-Ηη[Τ((ί))] 0) where L(t) is the thickness as a function of time t, a is the attenuation constant at 380nm, and T(t) is the transmission at time f.

T(t) may be taken to be the fractional intensity at f with the intensity when the substrate 2 0 was first submerged (t = 0) in the solution 208.

In another example (not shown in the figures), the in-situ measurement may be based on reflectance mode which enables the thickness of the film grown over a substantially non-transparent substrate to be measured.

Fig. 4A shows a cross-sectional view of a SEM image 400 of YGa-ZnO thin film growth 402 on a glass substrate 404 (for example, the substrate 210 of Figs. 2A, 2B and 3) in accordance with various embodiments disclosed herein. Fig. 4B shows an expanded view 420 of Fig. 4A. The scale bar of Fig. 4B is 500 nm. It can be seen from Figs. 4A and 4B that a relatively even and flat film 402 was obtained. The film 402 can be observed to be smooth. The quality of the produced film was smooth, fully coalesced surface with rms roughness, r of about 5-12 nm of textured single crystals, and large grain size of about 1 -3 pm. The grain sizes obtained were about 10 times larger (less defects) than those found in Ga:ZnO films prepared by known physical vapour deposition (PVD) and chemical vapour deposition (CVD) methods. Fig. 4C shows a HR-TEM image of a crystal column in accordance with various embodiments. The scale bar of Fig. 4C is 10 nm. The arrow indicates the crystallographic [0002] direction of the column. More importantly, it can be seen that the column is of single crystal orientation, and grows in the [0002] direction out-of-plane with respect to the substrate; thereby providing a textured single crystal structure. The inset of Fig. 4C shows a crystalline diffraction pattern of the YGa-ZnO thin film growth 402.

Smooth films are essential for producing highly transparent TCs as surface irregularities cause haze. Fig. 4D shows a graph 460 illustrating the transmission and haze measurements on an exemplary unpatterned film 462, 468, exemplary trenches 464, 470, and exemplary pillars 466, 472. It can be observed that transmission is maintained at about 85% for the structures of the unpatterned film 462, trenches 464 and pillars 466. Both trenches 470 and pillars 472 demonstrated minimal haze which is an average of about 8% more haze than unpatterned film 468 (electrodes), while maintaining the same level of total transmission. Thus, the structures on the conducting films enhance light scattering, without degrading the overall transparency.

F. FABRICA TION OF EXTENDED ELECTRODES

In example embodiments, extended electrodes include or consist of non- planar features that grow from the first layer film. In example embodiments, the bottom-up nature of this synthesis process allows for such features to be fabricated, giving it an advantage over other known methods of depositing films.

Fig. 5 shows a flow chart illustrating the process flow 500 of the synthesis of doped ZnO by the hydrothermal method and extended 3- dimensional (3D) electrodes (or projections) with the aid of lithography.

F-i) A film (or textured single crystal 502) was prepared on the substrate 506 having the seed layer 504 using the abovementioned steps (also reference to parts (l)-(lll) of Fig. 5). For example, the substrate 506 having the seed layer 504 may refer to the seeded substrate 210 of Figs. 2A and 2B. The textured single crystal 502 may refer to the film growth 402 of Fig. 4A). Lithography was used to selectively expose regions of the film 502 for subsequent growth, while the unexposed regions would not be subject to further growth. In lithography, a photoresist 508 (for example, poly(methyl methacrylate) PMMA) was firstly deposited over the film 502 (part (IV) of Fig. 5). A chrome mask 510 was used to provide the patterns to expose regions of growth (part (V) of Fig. 5). Exposed regions were treated with acetone and prepared as shown in part (VI) of Fig. 5. F-ii) The pattered film 508 of part (VI) of Fig. 5 was then immersed in a fresh growth solution (similar to the growth solution 208 of Figs. 2A, 2B and 3) to create the non-planar features, for example, extended electrodes 512 (part (VII) of Fig. 5). The layout of the features 512 was precisely positioned using lithography. The PMMA photoresist 508 was an appropriate choice as it is effective to demonstrate stability in high-pH growth solution.

F-iii) Thereafter, the formed doped ZnO material was removed from the growth reactor (for example, the growth system 200 of Figs. 2A and 2B) and the photoresist 508 was removed using acetone.

Fig. 6A shows a cross-sectional (perspective) view of a SEM image 600 of a YGa-ZnO thin film growth 602 on a substrate 604 with an extended electrode 606 (or may be referred to as a projection or a primary projection) in accordance with various embodiments disclosed herein. For example, the YGa- ZnO thin film growth 602 may in some embodiments, correspond to the YGa- ZnO thin film growth 402 or the textured single crystal 502 of Fig. 5, and the substrate 604 may in some embodiments, correspond to the glass substrate 404 of Fig. 4A. Fig. 6B shows an expanded view of an encircled area 6000 of Fig. 6A. It can be seen from Fig. 6B that there is a homojunction or a barrierless interface 620 between the extended electrode 606 and the film 602. G. FABRICATION OF ADDITIONAL EXTENDED BRANCHED

STRUCTURES

Other designs of additional extended electrodes may be fabricated based on secondary growth. Fig. 6C shows a S EM image 640 of a YGa-ZnO thin film growth 602 with an extended electrode 606 and branches 642. For example, the YGa-ZnO thin film growth 602 may in some embodiments, correspond to the textured single crystal 502 of Fig. 5 and the extended electrode 606 may in some embodiments, correspond to any one of the extended electrodes 512 of Fig. 5.

Fig. 6D shows an expanded view of an encircled area 6002 of Fig. 6C. It can be seen from Fig. 6D that there is a homojunction or a barrierless interface 660 between the extended electrode 606 and the film 602. G-i) The colloidal ZnO as prepared in the seeding step (B) as described above was spin coated onto the extended electrode 604.

G-ii) Subsequently the substrate (for example, the substrate 506 of Fig. 5) underwent the same growth process as described above to obtain the branched structures 606.

In exemplary embodiments, the disclosed method (hydrothermal process) advantageously offers process simplicity and application of low temperature. It also provides a low cost process. The synthesis process in accordance with various embodiments allows the growth of film on large lattice mismatch and amorphous substrates (for e.g., silicon and glass substrates). Various exemplary embodiments may provide growth of high quality film with simplicity in set-up, low operating cost and low operating temperature, while benefitting from the use of non toxic chemicals with an environmental friendly process. Being relatively cost effective and applications with low temperature allows various exemplary embodiments to be compatible to grow on flexible polymeric substrates. The process in accordance with various embodiments may also be potentially scalable for industrial production. Fig. 7 shows a block diagram illustrating an electrically conductive material 700 in accordance with various embodiments. The electrically conductive material 700 includes a substrate 702 in association with growth sites (not shown) and a continuous layer of electrically conductive crystalline metal oxide 704 grown from at least part of the growth sites.

For example, the electrically conductive material 700 may collectively refer to the substrate 506, the seed layer 504 and the textured single crystal 502 as shown in part (III) of Fig. 5. In other examples, the electrically conductive material 700 may collectively refer to the substrate 506, the seed layer 504, the textured single crystal 502 and the extended electrodes 512 as shown in part (VII) of Fig. 5.

In further examples, the electrically conductive material 700 may refer to an exemplary electrically conductive material 800 shown in Fig. 8. In Fig. 8, the electrically conductive material 800 includes an amorphous substrate 802 with a seed layer 804, adjacent to a textured single crystal co-doped YGa-ZnO layer 806 (or film). Extended electrodes 808 extend from a surface of the textured single crystal co-doped YGa-ZnO layer 806. For example, the extended electrodes 808 may extend substantially normal to the surface of the textured single crystal co-doped YGa-ZnO layer 806. As the extended electrodes 808 and the textured single crystal co-doped YGa-ZnO layer 806 have the same energy band, for example in this case, the extended electrodes are YGa-ZnO, each interface 812 therebetween is substantially a barrierless interface. Branches 810 were grown from the extended electrodes 808. For example, the branches 810 were grown substantially normal to a surface of the extended electrode 808. Similarly, as each branch 810 and the extended electrodes 808 have the same energy band, for example, the branch 810 is YGa-ZnO, each secondary interface 814 therebetween is substantially a barrierless interface.

APPLICATIONS

Embodiments of the methods disclosed herein provide a fast, efficient and cheap way of preparing an electrically conductive material. Embodiments of the disclosed methods also seek to overcome the problems of the scarcity of indium supply, to improve of transparency and conductivity of TCOs as well as to address the lack of environmental friendly, cheap and solution based TCOs.

Advantageously, embodiments of the methods disclosed herein is capable of preparing heteroepitaxial growth of textured single crystal singly/Co-doped Y-Ga-ZnO on large mismatch and amorphous substrates that does not require vacuum or clean room facilities.

Even more advantageously, as embodiments of the disclosed methods use low temperature, embodiments of the disclosed methods are compatible to grow on flexible polymeric substrates. Embodiments of the methods disclosed herein are also potentially scalable for industrial production due to its simplicity of operation. Patterned three dimensional (3D) homojunction transparent conducting electrodes can be fabricated in conjunction with the planar transparent conducting (TC) electrodes can also be produced by embodiments of the methods disclosed herein.

Advantageously, films with optimized surface areas and roughness for increased charge collector density, directed charge transport as well as enhanced light scattering capabilities may be obtained from embodiments of the methods disclosed herein. Accordingly, such films can be effectively used for photovoltaic applications.

In some embodiments, the methods disclosed herein are capable of utilizing ZnO to produce an electrically conductive material having a transparent conducting oxide layer. This is especially advantageous due to the abundance, low cost and non-toxic nature of ZnO.

Embodiments of the methods disclosed herein provide electrically conductive material having improved conductivity with co-doped ZnO material while maintaining the desirable transparency.

Advantageously, embodiments of the methods disclosed herein do not require the use of elaborate multi-steps processing methods, harsh chemicals and extensive equipment. Embodiments of the methods disclosed herein involve simple equipment with high ease of fabrication procedures. Even more advantageously, embodiments of the methods disclosed herein can be carried out in non-vacuum environment and at low temperature, which removes the requirement for expensive equipment that are compatible with vacuum environment and high temperature conditions. Embodiments of the disclosed electrically conductive material have low surface roughness of rms 5-12 nm. This is important in the use of electrodes for organic electronics where the roughness of any subsequently spin coated layer may be affected by the underlying electrode and excessive roughness may cause electrical shorting. It is also noted that rough films compromise transparency of TCOs.

Embodiments of the disclosed electrically conductive material overcomes the challenge of having the electrons travel through tortuous photoactive material path across typically 10-20 pm thick film to the FTO substrate for electron collection which often results in high recombination.

Embodiments of the disclosed electrically conductive material provide 2-in-1 electrodes which are made up of integrated planar TCO film with extended vertical and lateral electrodes. The planar and non planar structures may belong to the same material system grown using the same procedures in order to attain a barrier-less interface transport. High recombination occurs where there is a build-up of charges especially at poor material interface. Thus, embodiments of the disclosed electrically conductive material advantageously provide continuous homojunction conducting electrodes that does not have any interface discontinuity which in turn avoids fast recombination. Embodiments of the disclosed electrically conductive material also enhance the surface roughness of the TCO film through the 3D features. Embodiments of the disclosed electrically conductive material provide electrodes with 10-20 times the flat area through YGa.ZnO trenches and pillars. Advantageously, such extended electrodes can be used in conjunction with substrate-grown nanowires for chemical sensors with enhanced sensitivity or efficient photocatalytic applications.

In various embodiments disclosed herein, patterned 3D homojunction transparent conductive electrodes may be fabricated in conjunction with the planar TC electrodes to produce films with optimized surface areas and roughness for increase charge collector density and directed charge transport as well as enhanced light scattering capabilities specially demonstrated for photovoltaic applications. In these embodiments, the patterned homojunction based 3D electrodes may also show high transparency and low resistivity.

Embodiments of the disclosed methods may also be taken as a base procedure for applications to different structures. For example, the embodiments of disclosed methods may be extended to producing electrically conductive material having other structures other than simply primary and secondary projections based on the requirements of specific applications. Structural variations may be made to match various applications such as display, lighting, and solar cell.

Therefore, embodiments of the disclosed electrically conductive material may be advantageously used in solar photovoltaics, liquid crystal displays and Solid State Lighting (e.g. LEDs). As embodiments of the method disclosed herein do not require the use of sputtering or chemical deposition techniques, embodiments of the methods disclosed herein may avoid the problems faced when using sputtering or chemical vapour deposition techniques at an industrial level. It will be appreciated by a person skilled in the art that other variations and/or modifications may be made to the embodiments disclosed herein without departing from the spirit or scope of the disclosure as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

1. A method of preparing an electrically conductive material, the method comprising: providing a substrate in association with a plurality of growth sites; and

contacting at least part of the growth sites with a growth solution to cause a continuous layer of electrically conductive crystalline metal oxide to grow from the growth sites, wherein the growth solution comprises metal ions for growing the crystalline metal oxide.

2. The method of claim 1 , further comprising: providing the growth solution at a supersaturation temperature prior to the contacting step.

3. The method of claim 2, wherein the supersaturation temperature is no less than about 55 °C.

4. The method of any one of the preceding claims, wherein the crystalline metal oxide layer is substantially transparent to light.

5. The method of any one of the preceding claims, wherein the growth solution is provided at a volume that is dependent on an area of the continuous layer of electrically conductive crystalline metal oxide to be grown such that the ratio of the volume of growth solution provided to the area of the continuous layer of electrically conductive crystalline metal oxide to be grown is from about 15ml/cm² to about 30ml/cm², when the concentration of the growth solution is from about 25 to about 60 mM.

The method of any one of the preceding claims, wherein the growth sites and the crystalline metal oxide layer comprise a metal oxide selected from the group consisting of zinc oxide, tin oxide and titanium dioxide.

7. The method of any one of the preceding claims, wherein the growth solution further comprises one or more dopants.

8. The method of claim 7, wherein the one or more dopants comprise at least one of a rare earth metal, a metal from Group III of the periodic table of elements or both.

9. The method of any one of the preceding claims, wherein the contacting step comprises:

contacting the at least part of the growth sites with the growth solution for no more than about 3 hours.

10. The method of any one of the preceding claims, wherein the step of providing the substrate in association with the plurality of growth sites comprises:

forming a seed layer comprising the growth sites over the substrate.

11. The method of any one of the preceding claims, further comprising:

inhibiting the growth of the crystalline metal oxide layer when it reaches a predetermined thickness.

2. The method of any one of the preceding claims, wherein the substrate comprises at least one of an amorphous substrate, a crystalline substrate, a polycrystalline substrate or a mixture thereof. 3. The method of any one of the preceding claims, further comprising:

forming a barrier layer over the crystalline metal oxide layer; and

removing part of the barrier layer to expose a region of the underlying crystalline metal oxide layer to allow further growth of the crystalline metal oxide layer in the exposed region.

14. The method of claim 13, further comprising:

contacting the crystalline metal oxide layer with a growth solution to further grow a primary projection from the exposed region of the crystalline metal oxide layer.

15. The method of claim 14, further comprising:

forming a seed layer comprising a plurality of growth sites over the primary projection;

contacting the primary projection with a growth solution to further grow secondary projections from the primary projection.

16. The method of any one of the preceding claims, wherein the method is carried out under non-vacuum conditions.

17. An electrically conductive material comprising: a substrate in association with growth sites; and

a continuous layer of electrically conductive crystalline metal oxide grown from at least part of the growth sites.

18. The electrically conductive material of claim 17, wherein the crystalline metal oxide layer is substantially transparent to light.

19. The electrically conductive material of claim 17 or 18, wherein the crystalline metal oxide layer has a surface root mean square (rms) roughness in the range of from about 5 nm to about 12 nm.

20. The electrically conductive material of any one of claims 17 to 19, wherein the crystalline metal oxide layer has a grain size in the range of from about 500 nm to about 3 μηη.

21. The electrically conductive material of any one of claims 17 to 20, further comprising a seed layer adjacent to the substrate, wherein the seed layer comprises the growth sites.

22. The electrically conductive material of any one of claims 17 to 21 , wherein the crystalline metal oxide layer is co-doped with at least one of a rare earth metal, a metal from Group III of the periodic table of elements or both.

23. The electrically conductive material of any one of claims 17 to 22, wherein the crystalline metal oxide layer comprises one or more primary projections extending from the crystalline metal oxide layer; and wherein an interface between each of the one or more primary projections and the crystalline metal oxide layer comprises a homojunction.

24. The electrically conductive material of claim 23, wherein the crystalline metal oxide layer comprises one or more secondary projections extending from the one or more primary projections.

25. The electrically conductive material of any one of claims 17 to 24, wherein the substrate comprises at least one of an amorphous substrate, a crystalline substrate, a polycrystalline substrate or a mixture thereof.