WO2004094328A1

WO2004094328A1 - Coating compositions

Info

Publication number: WO2004094328A1
Application number: PCT/GB2004/001666
Authority: WO
Inventors: Peter Trenton Bishop; Patricia Ann Sutton
Original assignee: Johnson Matthey Public Limited Company
Priority date: 2003-04-17
Filing date: 2004-04-16
Publication date: 2004-11-04
Also published as: GB0308895D0

Abstract

A coating composition comprises a zinc carboxylate species and a liquid carrier. The zinc carboxylate species comprises a species according to formula (I):wherein Ar and Ar' are aryl groups which may be the same or different, substituted or unsubstituted. The compositions are suitable for the production of transparent conducting ZnO films and may contain dopants, polymers and additives. A method for the production of ZnO films on substrates is also disclosed, as are applications of such coated substrates.

Description

COATING COMPOSITIONS

This invention relates to coating compositions and in particular to coating compositions suitable for the formation of transparent conducting zinc oxide films. The invention also relates to methods for the formation of such films and applications thereof.

Transparent conducting oxide (TCO) coatings find use in a wide range of applications including solar cells, flat-panel display electrodes, optical waveguides, electromagnetic shielding, gas sensors, 'smart' displays and low emissivity and electrochromic windows. Ideally, such materials should be able to form as substantially transparent films with good electrical conductivity. This combination of high optical transparency and high electrical conductivity is not possible in an intrinsically stoichiometric material. With the exception of thin metallic films, the only way to create transparent conductors is to create electron degeneracy in a wide band gap material (>3eV) by controllably introducing non-stoichiometry and/or suitable dopant ions.

Such conditions may be obtained in oxides of cadmium, tin, indium, zinc and their alloys in thin film form. A variety of production techniques have been investigated, which have led to films being commercially produced with optical transparencies of 85- 90% and resistivities down to 10^"5 Ωcm.

The conducting properties of transparent oxides began to be investigated in the late 1940's, and concentrated on SnO₂ and CdO. In recent years the range of materials available has expanded considerably to include doped compounds such as SnO₂:F, ZnO:Al and In₂O₃:SnO₂ (ITO).

The principal techniques for TCO fabrication are as follows: - i) Evaporation of metallic or oxide sources; ii) Reactive and non-reactive forms of dc, rf, magnetron and ion-beam sputtering; iii) Chemical vapour deposition; iv) Spray pyrolysis; and v) Chemical solution growth (dip or spin coating).

Of these techniques, sputtering is the most commonly used to produce TCO films. Chemical solution synthesis has also been used to produce large area films of SnO and In₂O₃.

The optical and electronic properties are controlled by the deposition conditions and film characteristics. Good conductivity and high transparency are key desirable properties however, they are often related, leading to a trade-off between the two. For films deposited under optimised conditions both conductivity and transparency are strongly dependent on film thickness.

Within the field of transparent conducting oxides (TCO's), the majority of attention has been focussed on the In₂O₃:SnO₂ system (ITO) and SnO₂:F. Both of these materials are commercially available. However, doped ZnO based systems have the following advantages:

i) The precursor materials are cheap and abundant, which contrasts favourably with the inherently more expensive indium oxide based systems; ii) They are non-toxic, in contrast to cadmium containing materials; iii) A variety of synthetic routes are available, including large area chemical solution methods; iv) The band gap lies close to the short wavelength end of the visible spectrum.

Doped ZnO based systems have another advantage in terms of application to α- Si:H based solar cells. To date conventional materials such as ITO have been used as windows in such devices. The fabrication process for α-Si:H solar cells normally involves plasma enhanced chemical vapour deposition. The hydrogen plasma acts to reduce metal oxide materials to metals, thereby reducing cell efficiency. This degradation process for ITO and SnO₂:F occurs even under relatively benign plasma conditions. ZnO based films are, however, more stable to hydrogen plasma and as such are expected to bring significant benefits in terms of solar cell efficiency.

Accordingly, studies have been undertaken to assess the properties and fabrication routes to ZnO TCO materials containing Al, Ga, or In. Sputtering, although successful, is an inherently complex and expensive technique. Ohyama et al., Thin Solid Films 306 (1997) 78-85, have used sol-gel methods using zinc acetate as a starting material. A solution of zinc acetate in 2-methoxyethanol, stabilised by the addition of monoethanolamine, was used to deposit thin films which could be converted to ZnO on heating to between 400 and 800 °C, with optimum film properties achieved on firing at 600 °C

The present applicants have carried out a detailed study of the film forming abilities of the types of materials described by Ohyama et al. These studies were extended to include zinc acetate solutions stabilised with alternative amine ligands. Although in some cases, acceptable films were produced, the amine stabilised zinc acetate solutions were found to be very sensitive to atmospheric moisture, undergoing rapid hydrolysis on any contact with water. Hydrolysis of the solutions was found to have a detrimental affect on film formation.

A further disadvantage with the solutions used by Ohyama et al, is the toxicity of the 2-methoxyethanol used as a solvent.

The present invention addresses the aforementioned problems and provides an improved coating composition for the production of ZnO films. The composition is stable with respect to hydrolysis, has low temperature thermal decomposition characteristics and is non-toxic.

In accordance with a first aspect of the present invention, the composition comprises a zinc carboxylate species, a liquid carrier and a dopant; characterised in that the zinc carboxylate species comprises a species according to formula I: Zn(O₂CAr)(O₂CAr')

(I) wherein the aryl groups are the same or different, substituted or unsubstituted.

In one embodiment, each aryl group has the formula II:

(II) wherein the R_1; R_2; R_3j R₄ and R₅ are each selected from the group consisting of hydrogen, amino, linear or branched CrC₁₀ alkylamino, linear or branched CrC₁₀ dialkylamino, linear or branched C Cio alkyl, C₃-C₁₂ cycloalkyl, C -C₂₀ bicycloalkyl, C₅-C₂o polycycloalkyl, linear or branched Ci-Cio hydroxyalkyl, linear or branched C C₁₀ haloalkyl, linear or branched C C₁₀ perhaloalkyl, phenyl, linear or branched alkylsulfonyl, aryl, heteroaryl, tosyl, arylsulfonyl, acyl, linear or branched Cj-Cio alkylcarbonyl, benzoyl, aroyl, substituted aroyl, butoxycarbonyl, C^ o alkoxy, linear or branched Cι-C₁₀ alkylthio, linear O-acetyl, O-benzyl, O-substituted benzyl, O-tetrahydropyranyl, O- linear or branched Cj-C₁₀ alkylacetyl, O-benzoyl, O-aroyl, S-acetyl, S-benzyl, S-substituted benzyl, S- linear or branched Ci-Cio alkylacetyl, S-benzoyl, S-aroyl, hydroxyl, halogen, linear or branched C C₁₀ (C^Cs alkoxy) alkyl, formyl, acetyl, nitrile, carboxyl, linear or branched -Cio alkoxycarbonyl, nitro and amido or wherein neighbouring R groups form part of a further aromatic ring, optionally including one or more hetero atoms selected from N, S, O, further optionally substituted with any of the preceeding substirutents. For example, at least one aryl group can be a condensed polycyclic aromatic group, substituted or unsubstituted, for example naphthyl.

In specific embodiments the zinc carboxylate species is chosen from: Zn(O₂C-C6H - p-CMe₃)₂, Zn(O₂C-C₆H₄-2-OH)₂, Zn(O₂C-C₆H₄-p-OCH₃)₂, Zn(O₂C-C₆H4-p- OCH₂CH₃)₂, Zn(O₂C-C₆H₃-2-OH-3-CHMe₂)₂, Zn(O₂C-C₆H₃-3,5-CMe₃)₂, Zn(O₂C- C₆H₃-2,4-OH)₂ and Zn(O₂C-C₆H₂-3,4,5-OEt)₂, and in particular embodiments, the zinc carboxylate species is one of 2n(O₂C-C₆H₂-2-OH-3,5-CMe₃)₂₅ Zn(O₂C-C₆H₂-2-OH-3₅5- CHMe₂)₂ or Zn(O₂C-C₆H₂-4-OH-3,5-CMe₃)₂.

It will be understood that the composition may contain more than one different zinc carboxylate species.

The amount of zinc carboxylate species in the composition may be any suitable amount however generally, the zinc carboxylate species comprises between 0.1 and 30 wt% of the composition when expressed as an equivalent amount of ZnO. In a particular embodiment, the zinc carboxylate species comprises between 0.1 and 10 wt% of the composition when expressed as an equivalent amount of ZnO, for example 2.5 wt%.

A key benefit of the zinc carboxylate species of the present invention over amine stabilised zinc acetates, is their resistance to hydrolysis. Far less control over the humidity of the atmosphere during processing is thus required, leading to a simpler and less costly process. Furthermore, thermal decomposition behaviour is also superior, with complete decomposition occurring below 500 °C, and substantially lower still in many cases. Again, this provides cost benefits.

In one embodiment the liquid carrier is a solvent, preferably the solvent is a glycol ether such as, propylene glycol methyl ether acetate (PMA), propylene glycol methyl ether (Dowanol PM), propylene glycol propyl ether or dipropylene glycol dimethyl ether. Alternatively, other solvents such as acetone, alcohols, xylene and cyclohexanone may be used. Mixtures of two or more solvents are also suitable.

In another embodiment of the invention, the coating composition is an emulsion.

Dopants are conventionally used in the formation of TCO films to provide or enhance conductivity. The dopant generally comprises between 0.001 and 10 wt%, more typically, between 0.001 and 5 wt% expressed with respect to the amount of ZnO in the composition. Higher concentrations of dopant may also be used.

In a particular embodiment the dopant comprises at least one species of aluminium, gallium, indium, arsenic, yttrium, fluorine, boron and nitrogen. Some non-limiting examples of suitable dopants include Al(acac) , Al(O₂CC₆H₂-2-OH-3,5-CHMe₂)₃, aluminium nitrate, aluminium chloride, Ga(acac)₃, Ga(O₂CC₆H₂-2-OH-3,5-CHMe₂)₃, gallium nitrate, gallium chloride, and In(acac)₃, In(O₂CC₆H₂-2-OH-3,5-CHMe₂)₃, indium nitrate, indium chloride; although other species may also be used.

Although the formation of aluminium, gallium or indium doped ZnO films is a feature of the present invention, it is not intended to be limited thereto. Other metal or non-metal dopant species may be used to produce alternatively doped films, for example

As, Y, F, B, N, with dopants capable of providing 3+ ions being particularly preferred. More than one dopant species may be included in the composition.

The composition may further comprise one or more additives. Additives may include those which improve or alter the physical properties of the composition such as surfactants, rheology modifiers, adhesion strengthening additives and drying accelerators. Suitable surfactants will be known to those skilled in the art, for example carboxylates, phosphates, alcohols, amines, amides, polyethylene glycols, esters, ethoxylated surfactants and fluorinated surfactants. Alternative or additional additives may for example include, metals or other functional materials or pigments, such as gold based pigments. Amounts of additives used will be dependent on the intended application of the composition.

Compositions comprising a zinc carboxylate species, a liquid carrier and a dopant are generally of low viscosity. This makes them suitable for application by methods such as spin-coating and spraying. The viscosity and other properties of the compositions may be altered by the addition of polymers. Thus in a particular embodiment, the composition further comprises a polymer species. Compositions including polymer species, in addition to being suitable for application using spin-coating and spraying, may also be applied using techniques such as screen printing or ink-jet printing. When present, the polymer species may comprise any suitable amount of the composition. Precise amounts of polymer will depend on the specific application, the molecular weight of the polymer and its solubility in the chosen liquid carrier. Generally, the polymer may comprise between 0.01 and 60 wt% of the composition, typically between 0.01 and 30 wt% for example, between 0.5 and 20 wt%.

Suitably, the polymer comprises a poIy(meth)acrylate based polymer. Poly(meth)acrylates consist of long chains of small repeating units, the structure of which units is drawn below:

— CHR" — CH

CO₂R

where for acrylates, R" = H, for methacrylates, R" = methyl and R'" = alkyl or aryl groups.

Polymer chains can consist of only one repeating unit (homo-polymers), or consist of either blocks or random distributions of different units (co-polymers). Specific examples include; poly(ethyl acrylate-co-efhyl methacrylate-co-methyl methacrylate), poly(isobutyl methacrylate), poly(methyl methacrylate), poly(isobornyl methacrylate), poly(butyl methacrylate-co-methyl methacrylate) and poly(tert-butyl acrylate-co-ethyl acrylate-co-methacrylic acid). Other polymers will be known to those skilled in the art, such as poly(ethylene-co-vinyl acetate), poly(propylene carbonate) and poly(vinyl butyral). Mixtures of polymer species may also be used. The polymer species used should have similar thermal decomposition characteristics to the zinc carboxylate species. It is presently thought that traces of the polymer should not be retained in the ZnO film if good conductivity is to be achieved. In accordance with a second aspect of the present invention, a method of forming a coating comprising zinc oxide comprises the steps of:

(a) applying a coating composition in accordance with a first aspect of the invention to a substrate; and

(b) heat treating the substrate.

Suitable methods for applying the coating composition include spin coating, dip coating, spraying, aerosol, pyrosol, misting, brushing, screen printing or ink-jet printing, although other methods will be known to those skilled in the art. Compositions including polymer species are especially suitable for application by screen printing and ink-jet printing. Heretofore, is has been unknown to use screen printing or ink-jet printing for the formation of ZnO films. Screen printing and ink-jet printing have distinct advantages over application methods such as spin-coating and dip-coating. The composition can be applied only where required and in complex patterns or arrangements. This not only minimises waste but also obviates the need for post treatments to remove the coating from regions where it is not required; etching is commonly used for this purpose.

Generally, the substrate is substantially transparent. Most commonly, the substrate will be glass although transparent and semi-transparent crystal surfaces and high temperature polymers may equally be used. Although the method of the invention is primarily aimed at the production of transparent coatings on at least partially transparent substrates, it is not limited thereto. The substrate may equally be an opaque substrate for example, a metal or silicon. Substrates may be also pre-coated for example, with a metal or other type of coating.

A particular advantage of the method of the invention is the low thermal decomposition behaviour of the compositions used compared to those comprising amine stabilised zinc acetates. Generally, the substrate is heat treated at a temperature of between 200 and 600 °C, more typically, between 400 and 500 °C. Actual treatment temperatures and treatment times will vary dependent on the exact nature of the zinc carboxylate species, the polymer species, if present, and the nature of the substrate. Low treatment temperatures allow the formation of ZnO coatings on substrates which would be damaged by higher temperatures, such as low softening point glasses. Such substrates cannot be coated successfully using some zinc acetate based compositions. The process of the invention is also advantageous for substrates which can generally withstand high treatment temperatures. For example, the optical properties of some substrates may be compromised by high temperature treatment even though their gross physical properties may remain unaffected.

The method of the invention can be used to produce coatings which are substantially transparent. A completely transparent film will transmit 100% of visible light incident on it. Generally, films produced according to the invention will transmit at least 70%, typically at least 80% and more typically at least 90% of incident visible light.

In a particular embodiment, the films produced are also conducting, doped with for example, an aluminium, gallium or indium species if required.

In a further aspect, the invention provides a substrate coated using the method of the invention. Such coated substrates are suitable for incorporation into devices such as solar cells, flat-panel display electrodes, optical waveguides, gas sensors, 'smart' displays and low emissivity and electrochromic windows. The coated substrates may also be used for electromagnetic shielding and as IR reflectors. Other applications will be known to those skilled in the art.

In order that the invention may be more fully understood the following Examples are provided by way of illustration only and with reference to the accompanying drawing, in which:

Figure 1 is an electron microscopy image showing the uniform coating achieved using a screen printing formulation containing zinc carboxylate, solvent, dopant and polymer, after annealing at 500 °C. General Preparation of Zincffl) Carboxylate Compounds, Zn(O_jCAr)(Q₂CAr')

Preparation of Zinc Carboxylates by Zine Chloride Ronte

The desired carboxylic acid ligand, HO₂Ar (1 equivalent) was added slowly to a stirring solution of sodium hydroxide (1 equivalent) in water. The solution pH was carefully controlled to ensure the formation of a neutral solution. The above solution was added to a stirring solution of zinc chloride (0.5 equivalent) in water affording immediate precipitation of the desired product. The reaction was left to stir for 15-30 minutes before the product was collected by filtration. The product was dried at 100 °C prior to use. Microanalysis for preferred carboxylates is given in Table 1.

Table 1: Microanalysis of Zinc (II) Carboxylates, Zn(O₂CCR)₂

EXAMPLE 2 Preparation of Al(O2CCgH_r2-OH-3,5-CHMe₂)_j dopant

The carboxylic acid, HO₂CC₆H₂-2-OH-3,5-CHMe₂, (1 equivalent) was added to a stirring solution of sodium hydroxide (1 equivalent) in water. The solution pH was carefully controlled to ensure the resulting solution was close to neutral prior to reaction with aluminium chloride. The above solution was added dropwise to a stirring solution of AlCl₃.6H₂O (0.33 equivalent) in water. Addition caused the immediate precipitation of a white precipitate that was collected by filtration. The product was dried on the filter, and then at 70 °C overnight prior to use. EXAMPLE 3 - Solubility ofZnOD Carboxylates in Varied Solvents

The solubility of zinc carboxylates using substituted aryl carboxylic acids was assessed at a concentration of 10wt% within the solvents given in Table 2 below.

Formulations

The following examples are formulations, containing the preferred zinc(II) carboxylate compounds, which form high quality thin films of zinc oxide on thermal decomposition. All formulations were prepared with a "ZnO" concentration of 2.5 wt%. Films have been heat-treated and air annealed in a Nanetti Fast Fire Kiln and annealed under atmosphere in a tube furnace. Film thickness has been measured using a Laser profilometer and confirmed by Fast Ion Bombardment Transmission Electron Microscopy (FIB-TEM). Electrical measurements were made using a Jandel Engineering Four Point Conductivity Probe station in combination with a Keithley 237 high voltage source measurement and Keithley 181 nanovoltmeter.

Example 4 - 14 - Spin Coating Formulations

EXAMPLE 4

15.6 % Zn(O₂CC₆H₂-2-OH-3 ,5-CHMe₂)₂ 84.4 % 2-Methoxyethanol (Dowanol PM)

Thin layers of ZnO (~ 20 nm) were prepared by spin coating the formulation onto AF45 alkali-free glass (pre-heat treated at 400 °C, cleaned with ethanol) at 1500 rpm for 20 seconds, followed by heat treating the film at 400 °C for 10 minutes (10 °C / minute ramp-rate) in order to thermally decompose the zinc carboxylate to zinc oxide. This process was repeated until the desired number of layers (five) had been deposited. The film was then annealed at 500 °C for 1 hour in air to produce a highly transparent ZnO film of 100 nm thickness and poor conductivity.

EXAMPLE S

17.3 % Zn(O₂CC₆H₂-2-OH-3,5-CMe₃)₂

82.7 % 2-Methoxyethanol (Dowanol PM) A multiple layer film (five layers) of ZnO was prepared as for Example 4. On annealing at 500 °C for 1 hour in air a highly transparent zinc oxide film of - 100 nm thickness and of low conductivity was obtained.

EXAMPLE 6

17.3 % Zn(O₂CC₆H₂-4-OH-3,5-CMe₃)₂

82.7 % 2-Methoxyethanol (Dowanol PM)

A multiple layer film (five layers) of ZnO was prepared as for Example 4. On annealing at 500 °C for 1 hour in air a highly transparent zinc oxide film of ~ 100 nm thickness and of low conductivity was obtained.

EXAMPLE 7

15.6 % Zn(O₂CC₆H₂-2-OH-3 ,5-CHMe₂)₂

84.28 % 2-Methoxyethanol (Dowanol PM)

0.12 % Aluminium acetylacetonate (Aldrich)

Thin layers of aluminium doped zinc oxide (1.2 at % Al) were prepared by spin coating as described in Example 4. After the deposition and heat treatment of seven layers the film was annealed in air at 500 °C in air for 1 hour. The resultant film was again highly transparent and, in contrast to the undoped films, was found to be conductive (Table 3).

EXAMPLE 8

15.6 % Zn(O₂CC₆H₂-2-OH-3₅5-CHMe₂)₂

84.28 % Propylene glycol methyl ether acetate 0.12 % Aluminium acetylacetonate (Aldrich) Thin layers of aluminium doped zinc oxide (1.2 at % Al) were prepared by spin coating as described in Example 4. After the deposition and heat treatment of seven layers the film was annealed in air at 500 °C in air for 1 hour. The use of an alternative solvent within the formulation resulted in the deposition of a thinner film that was again both highly transparent and conductive (Table 3).

E AMP E S

17.3 % Zn(O₂CC₆H₂-2-OH-3,5-CMe₃)₂ 82.58 % 2-Methoxyethanol (Dowanol PM)

0.12 % Aluminium acetylacetonate (Aldrich)

Thin layers of aluminium doped zinc oxide (1.2 at % Al) were prepared by spin coating as described in Example 4. The spin speed was increased to 2500 rpm resulting in the deposition of thinner layers (- 13 nm). After the deposition and heat treatment of nine layers the film was annealed in air at 500 °C in air for 1 hour. The resultant film was again highly transparent and, in contrast to the undoped films, was found to be conductive (Table 3).

Table 3 : Film Properties of Spin Coated Films

EXAMPLE 10

17.3 % Zn(O₂CC₆H₂-2-OH-3,5-CMe₃)₂

75.08 % 2-Methoxyethanol (Dowanol PM) 0.12 % Aluminium acetylacetonate (Aldrich)

7.5 % Poly (ethyl acrylate-co-ethyl methacrylate-co-methyl methacrylate)

(Neocryl B-814)

The pofyacrylate polymer was incorporated into the formulation by pre- dissolving within Dowanol PM at a concentration of 30 wt%. The homogenous formulation was used to prepare a zinc oxide film by the method described in Example 4. Films were spun at 2500 rpm. After the deposition and heat treatment of five layers the film was annealed at 500 °C for 1 hour in air to produce a highly transparent film of similar conductivity to those prepared from non-polymer containing formulations (Table 4). The film was then annealed under N₂/H₂ (95:5 %) at 500 °C for 1 hour, further improving the film conductivity (Table 4).

EXAMPLE 11

17.3 % Zn(O₂CC₆H₂-2-OH-3,5-CMe₃)₂

75.08 % 2-Methoxyethanol (Dowanol PM)

0.12 % Aluminium acetylacetonate (Aldrich)

7.5 % Poly(isobutyl methacrylate) (Neocryl B-700)

The polyacrylate polymer was incorporated into the formulation by pre- dissolving within Dowanol PM at a concentration of 30 wt%. The homogenous formulation was used to prepare a zinc oxide film by the method described in Example 4. Films were spun at 2500 rpm. After the deposition and heat treatment of nine layers the film was annealed at 500 °C for 1 hour in air to produce a highly transparent film of similar conductivity to those prepared from non-polymer containing formulations (Table 4). The film was then annealed under N₂/H₂ (95:5 %) at 500 °C for 1 hour, further improving the film conductivity (Table 4). EXAMPLE 12

15.6 % Zn(O₂CC₆H₂-2-OH-3 ,5-CHMe₂)₂

76.78 % 2-Methoxyethanol (Dowanol PM) 0.12 % Aluminium acetylacetonate (Aldrich)

7.5 % Poly(ethyl acrylate-co-ethyl methacrylate-co-methyl methacrylate)

(Neocryl B-814)

The polyacrylate polymer was incorporated into the formulation by pre- dissolving within Dowanol PM at a concentration of 30 wt%. The homogenous formulation was used to prepare a zinc oxide film by the method described in Example 4. Films were spun at 1500 rpm. After the deposition and heat treatment of seven layers the film was annealed at 500 °C for 1 hour in air to produce a highly transparent film of similar conductivity to those prepared from non-polymer containing formulations (Table 4).

EXAMPLE 13

15.6 % Zn(O₂CC₆H₂-2-OH-3,5-CHMe₂)₂ 79.78 % 2-Methoxyethanol (Dowanol PM)

0.12 % Aluminium acetylacetonate (Aldrich)

4.5 % Poly(methyl methacrylate) (Neocryl B-728)

The polyacrylate polymer was incorporated into the formulation by pre- dissolving within Dowanol PM at a concentration of 30 wt%. The homogenous formulation was used to prepare a zinc oxide film by the method described in Example 4. Films were spun at 1500 rpm. In contrast to previous examples, slightly hazy films were obtained prior to firing, showing a lower level of compatibility between the zinc compound and this polymer. After the deposition and heat treatment of seven layers the film was annealed at 500 °C for 1 hour in air to produce a slightly hazy film of similar conductivity to those prepared from non-polymer containing formulations (Table 4). EXAMPLE 14

15.6 % Zn(O₂CC₆H₂-2-OH-3,5-CHMe₂)₂

54.08 % Dipropylene glycol dimethyl ether (Proglyde DMM) 25.0 % 2-Methoxyethanol (Dowanol PM)

0.32 % Indium acetylacetonate (Aldrich)

5.0 % Poly (ethyl acrylate-co-ethyl methacrylate-co-methyl methacrylate)

(Neocryl B-814)

The polyacrylate polymer was incorporated into the formulation by pre- dissolving within Dowanol PM at a concentration of 20 wt%. The homogenous formulation was used to prepare a zinc oxide film by the method described in Example 4. Films were spun at 1000 rpm. After the deposition and heat treatment of eight layers the film was annealed at 500 °C for 1 hour in air to produce a highly transparent, conductive film (Table 4).

Table 4: Film Properties of Spin Coated Films Examples 15 — 21 - Screen Printing Formulations

ElIAMPLE 15

15.6 % Zn(O₂CC₆H₂-2-OH-3 ,5-CHMe₂)₂

43.8 % Dipropylene glycol dimethyl ether (Proglyde DMM)

19.9 % 2-Methoxyethanol (Dowanol PM)

20.7 % Poly(ethyl acrylate-co-ethyl methacrylate-co-methyl methacrylate)

(Neocryl B-814)

The polyacrylate polymer was incorporated into the formulation by pre-dissolving into dipropylene glycol dimethyl ether at a concentration of 40 wt%. The formulation was hand printed through a 390# mesh polyester screen onto AF45 glass and the resulting organic film was heat treated at 400 °C (10 °C / minute ramp-rate) and then annealed at 500 °C for 1 hour in air to produce a highly transparent film with a pale blue colouration at certain angles of view that was of low conductivity.

EXAMPLE 16

15.6 % Zn(O₂CC₆H₂-2-OH-3,5-CHMe₂)₂

43.68 % Dipropylene glycol dimethyl ether (Proglyde DMM)

19.9 % 2-Methoxyethanol (Dowanol PM)

0.12 % Aluminium acetylacetonate (Aldrich)

20.7 % Poly(ethyl acrylate-co-ethyl methacrylate-co-methyl methacrylate) (Neocryl B-814)

The formulation was prepared and deposited onto AF45 glass as described in Example 15. The film obtained after air annealing at 500 °C was visually similar to that obtained for Example 15, but was found to be conductive (Table 5). SEM analysis of the fired film suggested a film thickness of 80 - 100 nm from a single screen printed layer. Conductivity was shown to be similar to that obtained from films prepared by the deposition of multiple layers via spin coating. Thicker films were prepared from this formulation by printing and heat-treating five layers. The films were then annealed in air at 500 °C for 1 hour. Multiple layer films were found lo be of similar resistivity to the single layer films (Table 5). Analysis of the film prepared by multiple layer deposition by FIB-TEM revealed a film of 350 nm thickness consisting of densely packed ZnO crystallites (20 -50 nm), see Figure 1. The film was homogenous with no evidence of the film having been prepared by the deposition of five separate layers (Figure 1).

The conductivity of the multiple layer film was improved by further annealing under N₂/H₂ at 400 °C for 1-2 hours (Table 5).

EXAMPLE 17

15.6 % Zn(O₂CC₆H₂-2-OH-3 ,5-CHMe₂)₂ 43.68 % Dipropylene glycol dimethyl ether (Proglyde DMM)

19.9 % 2-Methoxyethanol (Dowanol PM)

0.12 % Aluminium acetylacetonate (Aldrich)

20.7 % Poly(isobutyl methacrylate) (Neocryl B-700)

The formulation was prepared and deposited onto AF45 glass as described in

Example 15. The film obtained after air annealing at 550 °C for 1 hour was visually similar to that obtained for Example 10, but was found to be conductive (Table 5). SEM analysis of the fired film suggested a film thickness of 80 - 100 nm from a single screen printed layer. EXAMPLE 18

15.6 % Zn(O₂CC₆H₂-2-OH-3,5-CHMe₂)₂

43.68 % Dipropylene glycol dimethyl ether (Proglyde DMM)

19.9 % 2-Methoxyethanol (Dowanol PM) 0.12 % Aluniinium acetylacetonate (Aldrich)

20.7 % Poly(butyl-co-methacrylate), mol. wt 150,000 (Aldrich) The formulation was prepared and deposited onto AF45 glass as described in

Example 15. The film obtained after air annealing at 525 °C for 1 hour was visually similar to that obtained for Example 15, but was found to be conductive (Table 5). SEM analysis of the fired film suggested a film thickness of 80 - 100 nm from a single screen printed layer.

EXAMPLE 19

17.3 % Zn(O₂CC₆H₂-4-OH-3,5-CMe₃)₂ 41.98 % Dipropylene glycol dimethyl ether (Proglyde DMM)

19.9 % 2-Methoxyethanol (Dowanol PM)

0.12 % Aluminium acetylacetonate (Aldrich)

20.7 % Poly(ethyl acrylate-co-ethyl methacrylate-co-methyl methacrylate)

(Neocryl B-814)

The formulation was prepared and deposited onto AF45 glass as described in

Example 15. The film obtained after air annealing at 500 °C for 1 hour was visually similar to that obtained for Example 15. The film was found to be conductive (Table 5).

EXAMPLE 20

17.3 % Zn(O₂CC₆H₂-2-OH-3,5-CMe₃)₂

41.98 % Dipropylene glycol dimethyl ether (Proglyde DMM)

19.9 % 2-Methoxyethanol (Dowanol PM) 0.12 % Aluminium acetylacetonate (Aldrich)

20.7 % Poly(ethyl acrylate-co-ethyl methacrylate-co-methyl methacrylate)

(Neocryl B-814)

The formulation was prepared and deposited onto AF45 glass as described in Example 15. In contrast to results obtained from the other zinc carboxylates in combination with this polymer the film was hazy and of negligible conductivity. EXAMPLE 21

15.6 % Zn(O₂CC₆H₂-2-OH-3,5-CHMe₂)₂

43.1 % Dipropylene glycol dimethyl ether (Proglyde DMM)

19.8 % 2-Methoxyethanol (Dowanol PM)

0.23 % Gallium acetylacetonate (Aldrich)

0.5 % Byk 354

20.7 % Poly(ethyl acrylate-co-ethyl methacrylate-co-methyl methacrylate)

(Neocryl B-814)

The formulation was prepared and deposited onto AF45 glass as described in Example 15. Thicker films of 300 nm were prepared from this formulation by printing and heat treating five layers, followed by annealing in air at 500 °C for 1 hour. This resulted in conductive, transparent films. The resistivity was lowered further by annealing the film under N₂ H₂ (Table 5).

Table 5: Film Properties of Screen Printed Films EXAMPLE 22

Inkjet Printing Formulation

15.6 % Zn(O₂CC₆H₂-2-OH-3 ,5-CHMe₂)₂ 84.28 % 2-Methoxyethanol (Dowanol PM)

0.12 % Aluminium acetylacetonate (Aldrich)

The formulation (same as for Example 6) was deposited onto AF45 glass using a TRIDENT Ultrajet II 96 printhead. The film was heat treated at 400 °C for 10 minutes (20 °C / minute ramp-rate) and then annealed at 500 °C for 1 hour in air. The film was highly transparent and found to be conductive (Table 6). The film thickness was shown to be - 87 nm by FIB-TEM analysis which is similar to layer thickness deposited by a single screen print.

Table 6: Film Properties of Inkjet Printed Films

EXAMPLE 23 Emulsion Formulation

16.7 % Zn(O₂CC₆H₂-3 ,4,5-OMe)₂

58.3 % 2-methoxyethanol (Dowanol PM)

20.0 % dipropylene glycol dimethyl ether

5.0 % poly(ethyl acrylate-co-ethyl methacrylate-co-methyl methacrylate)

(Neocryl B-814)

The zinc compound was dispersed within the polymer solution using an ah driven Silverson mixer for 10 minutes. The formulation was then spin coated onto AF45 glass at 1000 rpm for 20 seconds. The resultant film was heat treated at 400 °C for 10 minutes (30 °C / minute ramp-rate) in order to thermally decompose the zinc carboxylate to zinc oxide. This process was repeated until the desired number of layers had been deposited. The film was then annealed at 500 °C for 1 hour in air to produce a ZnO film of low transparency compared to those prepared from more soluble precursors.

Claims

1. A coating composition, the composition comprising a zinc carboxylate species, a liquid carrier and a dopant; characterised in that the zinc carboxylate species comprises a species according to formula I:

Zn(O₂CAr)(O₂CAr')

2. A composition according to claim 1, each aryl group has the formula II:

(II) wherein the R_1; R₂, R₃, and R₅ are each selected from the group consisting of hydrogen, amino, linear or branched Ci-Cio alkylamino, linear or branched Cι-Cιo dialkylamino, linear or branched Ci-Cio alkyl, C₃-C₁₂ cycloalkyl, C₄-C₂₀ bicycloalkyl, C5-C₂o polycycloalkyl, linear or branched Cι-C₁₀ hydroxyalkyl, linear or branched Ci-Cio haloalkyl, linear or branched Cι-Cιo perhaloalkyl, phenyl, linear or branched alkylsulfonyl, aryl, heteroaryl, tosyl, arylsulfonyl, acyl, linear or branched CpCio alkylcarbonyl, benzoyl, aroyl, substituted aroyl, butoxycarbonyl, Ci-Cio alkoxy, linear or branched - o alkylthio, linear O-acetyl, O-benzyl, O-substituted benzyl, O-tetrahydropyranyl, O- linear or branched Ci-Cio alkylacetyl, O-benzoyl, O-aroyl, S-acetyl, S-benzyl, S-substituted benzyl, S- linear or branched CpCio alkylacetyl, S-benzoyl, S-aroyl, hydroxyl, halogen, linear or branched Ci-Cio (CpCs alkoxy) alkyl, formyl, acetyl, nitrile, carboxyl, linear or branched Ci-Cio alkoxycarbonyl, nitro and amido, or wherein neighbouring R groups form part of a further aromatic ring, optionally including one or more hereto atoms selected from N, S, O, further optionally substituted with any of the preceeding substitutents.

3. A composition according to claim 1 or 2, wherein at least one aryl group is a condensed polycyclic aromatic group, substituted or unsubstituted, optionally naphthyl.

4. A composition according to claim 1, 2 or 3, wherein the zinc carboxylate species is chosen from the group consisting of Zn(O₂C-C₆H₄-p-CMe₃)₂, Zn(O₂C-C₆H₂-2-OH-3,5-CMe₃)₂, Zn(O₂C-C₆H₄-2-OH)₂, Zn(O₂C-C₆H4-p-OCH₃)₂, Zn(O₂C-C6H₄-p-OCH₂CH₃)₂, Zn(O₂C-C₆H₂-2-OH-3,5-CHMe₂)₂, Zn(O₂C-C₆H₂-4-OH- 3,5-CMe₃)₂, Zn(O₂C-C₆H₃-2-OH-3-CHMe₂)₂, Zn(O₂C-C₆H₃-3,5-CMe₃)₂, Zn(O₂C-C₆H₃- 2,4-OH)₂ and Zn(O₂C-C₆H₂-3,4,5-OEt)₂.

5. A composition according to any of claims 1 to 4, wherein the zinc carboxylate species is chosen from the group consisting of Zn(O₂C-C₆H₂-2-OH-3,5-CMe₃)₂,

Zn(O₂C-C₆H₂-2-OH-3,5-CHMe₂)₂ and Zn(O₂C-C₆H₂-4-OH-3,5-CMe₃)₂.

6. A composition according to any preceding claim, wherein the zinc carboxylate species comprises 0.1 and 30 wt% of the composition when expressed as an equivalent amount of ZnO

7. A composition according to any preceding claim, wherein the liquid carrier is a solvent.

8. A composition according claim 7, wherein the solvent comprises at least one of a glycol ether, acetone, an alcohol, xylene and cyclohexanone.

9. A composition according to any of claims 1 to 6, wherein the composition is an emulsion.

10. A composition according to any preceding claim, wherein the dopant comprises between 0.001 and 10 wt% expressed with respect to the amount of ZnO in the composition

11. A composition according to claim 10, wherein the dopant comprises at least one of aluminium, gallium, indium, arsenic, yttrium, fluorine, boron and nitrogen

12. A composition according to claim 10 or 11, wherein the dopant is chosen from the group consisting of: Al(acac)₃, Al(O₂CC₆H₂-2-OH-3,5-CHMe₂)₃, aluminium nitrate and aluminium chloride.

13. A composition according to claim 10 or 11, wherein the dopant is chosen from the group consisting of Ga(acac)₃, Ga(O₂CC₆H₂-2-OH-3,5-CHMe₂)₃, gallium nitrate and gallium chloride.

14. A composition according to claim 10 or 11, wherein the dopant is chosen from the group consisting of In(acac)₃, In(O CC₆H₂-2-OH-3,5-CHMe₂)₃, indium nitrate and indium chloride.

15. A composition according to any preceding claim further comprising one or more additives chosen from the group consisting of surfactants, rheology modifiers, adhesion strengthening additives, drying accelerators and pigments.

16. A composition according to any preceding claim further comprising a polymer species.

17. A composition according to claim 16, wherein the polymer species comprises from between 0.01 and 60 wt% of the composition.

18. A composition according to claim 16 or 17, wherein the polymer species comprises a poly(meth)acrylate.

19. A composition according to claim 16 to 17, wherein the polymer species is chosen from the group consisting of poly(ethyl acrylate-co-ethyl methacrylate-co-methyl methacrylate), poly(isobutyl methacrylate), pory(methyl methacrylate), poly(isobornyl methacrylate), poly(butyl methacrylate-co-methyl methacrylate), poly(tert-butyl acrylate- co-ethyl acrylate-co-methacrylic acid), poly(ethylene-co-vinyl acetate), poly(proρylene carbonate) and poly (vinyl butyral).

20. A method of forming a coating comprising zinc oxide, the method comprising the steps of: (a) applying a composition according to any preceding claim to a substrate, and (b) heat treating the substrate.

21. A method according to claim 20, wherein the composition is applied to the substrate by spin coating, dip coating, spraying, aerosol, pyrosol, misting, brushing, screen printing or ink-jet printing.

22. A method according to claim 20 or claim 21, wherein the substrate is substantially transparent.

23. A method according to any of claims 20 to 22, wherein the substrate is glass.

24. A method according to any of claims 20 to 23, wherein the substrate is heat treated at a temperature between 200 °C and 600 °C.

25. A method according to any of claims 20 to 24, wherein the coating is substantially transparent and conducting.

26. A substrate coated using a method according to any of claims 20 to 25.

27. A flat-panel display electrode incorporating a substrate according to claim 26.

8. A solar cell incorporating a substrate according to claim 26.