GB2428689A - Process for preparing transparent conducting metal oxides - Google Patents

Abstract

A process for coating a substrate with a transparent conducting metal oxide film, comprises coating the substrate with a composition comprising a metal carboxylate species and a liquid carrier, heating the coated substrate to evaporate the liquid carrier and form a film of transparent conducting metal oxide and irradiating the film with a laser. In preferred embodiments the metal carboxylate species comprise arylic zinc or indium carboxylate. A flat-panel display electrode or a solar cell incorporating a transparent conducting oxide.

Description

CBC 1748 2428689

PROCESS FOR PREPARING TRANSPARENT CONDUCTING METAL

OXIDES

This invention relates to a process for making transparent conducting metal oxides using laser irradiation. This invention also relates to applications of transparent conducting oxides.

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 l0 fcm.

The conducting properties of transparent oxides began to be investigated in the late 1940's, and concentrated on Sn02 and CdO. In recent years the range of materials available has expanded considerably to include doped compounds such as Sn02:F, ZnO:Al and 1n203:Sn02 (ITO).

The techniques used for TCO fabrication have various advantages and disadvantages associated with them. Furthermore, the choice of technique is key to the optical and electronic properties as these are controlled by the deposition conditions and resulting film characteristics. Good conductivity and high transparency are the primary desirable properties, however, they are often related, leading to a trade-off between the two. Both conductivity and transparency are strongly dependent on film thickness.

The principal techniques for TCO fabrication are as follows: - i) Evaporation of metallic or metal oxide sources; ii) Reactive and nonreactive 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).

The metallic or metal oxide sources used for the evaporation technique are expensive and the process is inefficient in that a significant proportion of the source does not reach the substrate, instead being lost to the atmosphere. Sputtering is the most commonly used of these techniques, producing dense and homogeneous TCO films.

However, although successful this is an inherently complex and expensive technique, as is chemical vapour deposition. Spray pyrolysis is an inexpensive technique using simple wet chemical precursor formulations, however it can be difficult to ensure that any dopant present in the formulation is present in the same concentration in the film.

Additionally, spray pyrolysis still requires the post treatment of the TCO layer by etching, an expensive and time consuming process.

Chemical solution synthesis has also been used to produce large area films of Sn02 and 1n203, whilst Ohyama et al., Thin Solid Films 306 (1997) 78, have used sol-gel methods using zinc acetate as a starting material to prepare ZnO films. The present applicants have previously carried out a detailed study of the film forming abilities of the types of materials described by Ohyama et al. and, while in some cases acceptable films were produced, the solutions were found to be very sensitive to atmospheric moisture. In particular, the solutions underwent rapid hydrolysis on any contact with water, which 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.

Imai et al. (Sol-Gel Science Technol. 13 (1998) 991) prepared transparent, crystalline ITO films with a low electrical resistance by laser irradiation of sol-gel derived indium I tin alkoxide films. Comparison with films formed through thermal treatments suggested that atomic displacements were induced by laser irradiation. ZnO films were prepared by laser irradiation of sol-gel derived zinc acetate films by Nagase et al., Thin Solid Films 357 (1999) 151. As with the ITO films, these films demonstrated a decreased electrical resistivity in comparison to ZnO films formed after thermal treatment alone. This is thought to result from laser irradiation creating oxygen vacancies that enable oxygen conduction through the lattice.

We have now developed a process for the manufacture of transparent conducting metal oxides, which are both highly conductive and highly transparent.

In accordance with a first aspect of the present invention, the process comprises coating a substrate with a transparent conducting metal oxide film, which process comprising coating the substrate with a composition comprising a metal carboxylate species and a liquid carrier, heating the coated substrate to evaporate the liquid carrier and form a film of transparent conducting metal oxide and irradiating the film with a laser.

This process has the advantage that it can be used to prepare films at a lower temperature than might be required using thermal treatment alone making it suitable for use with a greater range of substrates, additionally the laser irradiation may be tuned so that it is not absorbed by the substrate resulting in localised heating of only the deposited coating. Although the preparation of films using laser irradiation involves an extra step in comparison to thermal treatment alone, the process does not add significantly to the time taken to prepare the transparent conducting oxides as the laser irradiation step only takes approximately 1 minute to carry out.

In one embodiment, the metal carboxylate species comprises one or more species selected from the group consisting of zinc carboxylate species and indium carboxylate species. It will be understood that the metal carboxylate species may consist of more than one different zinc and/or indium carboxylate species.

ZnO and InZnO based systems formed the basis of the present applicant's WO 2004/094328 and GB Application No. 0508597.2, which claim coating compositions that form films with low resistivities and demonstrate improved stability with respect to hydrolysis, whilst also having low temperature thermal decomposition characteristics.

In specific embodiments, 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 specific embodiments, the indium carboxylate species comprises a species according to formula IT: In(O2CAr)(O2CAr')(O2CAr") (II) wherein the aryl groups are the same or different, substituted or unsubstituted.

In one embodiment the liquid carrier is a solvent, commonly 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.

In one embodiment the coating composition may additionally comprise a dopant.

Dopants are conventionally used in the formation of TCO films to provide or enhance conductivity.

In a particular embodiment the dopant comprises at least one species of tin, titanium, vanadium, molybdenum, tungsten, cerium, aluminium, gallium, arsenic, yttrium, fluorine, boron and nitrogen. Some non-limiting examples of suitable dopants include Al(acac)3, Al(O2CC6I-12-2-OH-3,5- CHMe2)3, aluminium nitrate, aluminium chloride, Ga(acac)3, Ga(02CC6H2-2- OH-3,5 -CHMe2)3, gallium nitrate and gallium chloride; although other species may also be used. 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.

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. 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. The polymer species used should have similar thermal decomposition characteristics to the metal carboxylate species. It is presently thought that traces of the polymer should not be retained in the film if good conductivity is to be achieved.

Compositions including polymer species are especially suitable for application by screen printing and ink-jet printing, but may also be applied by spin-coating and spraying. Other suitable methods for applying the coating composition include dip coating, aerosol, pyrosol, misting, brushing, although other methods will be known to those skilled in the art. 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 process of the invention is primarily aimed at the manufacture 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 also be pre-coated, for example with a metal or other type of coating.

Generally, the coated substrate is heated at a temperature between 300 C and 600 C, typically, between 400 and 500 C. Actual treatment temperatures and times will vary dependent on the nature of the metal carboxylate species, the polymer species, if present, and the nature of the substrate. Laser irradiation offers the advantage of being able to produce transparent conducting oxides at lower temperatures than through thermal treatment alone, and allows the formation of transparent conducting oxide coatings on substrates that would be damaged by the high temperatures needed to form them by thermal treatment alone. The process of the invention is also advantageous for substrates which can generally withstand high treatment temperatures, but whose optical properties may be compromised even though their gross physical properties may remain unaffected.

In one embodiment, the film is less than 200 nm in thickness, optionally of between 100 and 150 rim in thickness. It is thought that the invention is particularly suited to laser irradiation of films of this range of thicknesses because the laser is better able to penetrate through the films enabling structural changes to occur throughout.

In another embodiment, the coating composition is applied to the substrate no more than three times (with intermediate drying steps) prior to irradiation.

Generally, thermally treated films made from a single, thicker deposition of coating composition onto a substrate are significantly less conductive than those prepared from thinner multiple layers. However, we have found that laser irradiated films made from a single deposition are highly conductive and have excellent transparency. Therefore in another embodiment, the coating composition is applied to the substrate only once prior to laser irradiation.

In a further embodiment, an additional process step of annealing the substrate in air and/or in a N2/H2 atmosphere may be inserted between the step of heating the coated substrate and the step of irradiating the film with a laser. Although this additional pre- treatment step is not essential to produce highly conductive films it has been shown to lower the resistivity of the transparent conducting metal oxide films formed after laser annealing still further in certain cases.

The laser irradiating step is generally carried out at an energy fluence of between and 200 mJ/cm2. Higher laser fluences improve the conductivity of the films more, but also affect the surface of the films more leading to reduced transparency. Therefore, a balance needs to be struck between film conductivity and transparency in each case depending on the characteristics of the coating composition used.

A transparent conducting oxide produced according to the invention will exhibit a resistivity of less than 0.4 = =cm, more commonly a resistivity of less than 0.04 acm.

In a further aspect, the invention provides a transparent conducting oxide 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 transparent conducting oxides 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 fblly understood the following non- limiting Examples are provided by way of illustration only: General Preparation of Metal Carboxylate Compounds:

EXAMPLE 1

Synthesis of Zinc(II) Carboxylate Compounds. Zn(O The desired carboxylic acid ligand, HO2CR (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 one zinc carboxylate is given in Table I. Compound C,H microanalysis found (expected) Zn(02CC6H2-2-OH-3,5-CHMe2)2 C 61.5 (61.5) N 6.7 (6.7) Table 1: Microanalysis of Zinc (11) Carboxylates, Zn(O2CCR)2

EXAMPLE 2

Synthesis of Indium(III) Carboxylate Compounds, 1n(OR) The desired carboxylic acid ligand, HO2CR (I 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 indium chloride (0.333 equivalents) 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 one indium carboxylate is given in Table 2.

Compound C, H microanalysis found (expected) O2CC6H2-2-OH-3,5-CHMe2)3 C 59.4 (60.1) N 6.4 (6.5) Table 2: Microanalysis of Indium (Ill) Carboxylates, n(O2CCR)3 Formulations: Examples 3, 4 and 5 describe methods of making high quality films of ZnO or ZnkIn2O3+k with formulations containing the zinc and indium carboxylates from Examples I and 2, in various ratios of Zn:In, for subsequent laser irradiation (described in Examples 6, 7 and 8). Films of various overall film thicknesses (variation of number of layers) and layer thicknesses (variation of screen used) were prepared by a). thermal treatment only, b) . thermal treatment and air annealing, or c). thermal treatment, air annealing and N2/l-12 atmosphere annealing. Films were air annealed in a Nanetti Fast Fire Kiln and N2/H2 atmosphere annealing was done in a tube furnace.

Film thickness has been measured using a Laser Profilometer and film structure has been studied using 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.

All formulations given below are screen printing formulations. Formulations suitable for spin coating can be made by removal of the polyacrylate from the formulation.

EXAMPLE 3

Preparation of Zn1n204 Films 2.65 % Zn(02CC6H2-2-O1-l-3,5-CHMe2)2 8.12 % Ifl(02CC6H2-2-OH-3,5-Cl-lMe2)3 22 % Poly(ethyl acrylate-co-ethyl methacrylate-co-methyl methacrylate) (Neocryl B-814) 20 % 2-Methoxyethanol (Dowanol PM) 46.53 % Dipropylene glycol dimethyl ether (Proglyde DMM) 0.7% Byk354 The polyacrylate polymer was incorporated into the formulation by pre- S dissolving into dipropylene glycol dimethyl ether at a concentration of 40 wt%. The formulation was prepared simply by dissolving the indium and zinc carboxylate compounds into the solvents with stirring. The formulation was printed onto AF45 alkali-free glass through 380-34Y, 380- 31Y and 195T mesh polyester screens i). x 3 and x 5 times, ii). x2 times, and iii). xl time respectively. The resulting organic film was heat- treated at 500 C for 10 minutes (30 C/min ramp-rate) between each printing step, such that this process of layer deposition and thermal decomposition produced a film of the desired thickness. This resulted in conductive, highly transparent films. The conductivity of some of the films was improved by further annealing under N2/H2 at 400 C for 1-2 hours.

EXAMPLE 4

Preparation of ZnQL Films 6.31 % Zn(02CC61-12-2-OH-3,5-CHMe2)2 4.85 % Tn(02CC6H2-2-OH-3,5-CHMe2)3 22 % Poly(ethyl acrylate-co-ethyl methacrylate-co-methyl methacrylate) (Neocryl B-814) % 2-Methoxyethanol (Dowanol PM) 46.14 % Dipropylene glycol dimethyl ether (Proglyde DMM) 0.7% Byk354 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 prepared simply by dissolving the indium and zinc carboxylate compounds into the solvents with stirring. The formulation was printed onto AF45 alkali-free glass through 380-34Y, 3803lY and 195T mesh polyester screens I). x 3, x 5 and x7 times, ii). x 2 times, and iii). x I time respectively. The resulting organic film was heat-treated at 500 C for 10 minutes (30 C/min ramp-rate) between each printing step, such that this process of layer deposition and thermal decomposition produced a film of the desired thickness. This resulted in conductive, highly transparent films. The conductivity of some of the films was improved by further annealing under N2/H2 at 400 C for 1-2 hours.

EXAMPLE 5

Preparation of Ga doped ZnO Films 15.6% Zfl(02CC6H2-2-Ol-1-3,5C1-[Me2), 43.17% 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 polyacrylate polymer was incorporated into the formulation by pre-dissolving into dipropylene glycol dimethyl ether at a concentration of 40 wt%. The formulation was prepared simply by dissolving the zinc carboxylate and gallium compounds into the solvents with stirring. The formulation was printed onto AF45 alkali-free glass through 380-34Y and 195T mesh polyester screens i). x3 and x6 times, and ii). xl time respectively. The resulting organic films were heat treated at 400 C (10 C I minute ramp-rate) and then annealed at 500 C for 1 hour in air between each printing step, such that this process of layer deposition and thermal decomposition produced a film of the desired thickness. This resulted in conductive, transparent films.

Laser Irradiation of the Films: Laser irradiation of films prepared according to Examples 3, 4 and 5 was carried out using a 308 nm Excimer laser, using a 20 ns pulse length and a frequency of'- 1 Hz.

The number of shots was set at four with an area of 6 mm2, with the samples "stitched" over, so that each area of the sample received at least four shots during the laser annealing process. The fluence (energy) of the laser was varied between 50 and 260 mJ/cm2 so as to assess its influence on film structure and properties.

EXAMPLE 6

Laser Irradiation of Zn1n204 Films Three layer screen printed films (38034Y screen) prepared by a) thermal treatment only, b) thermal treatment and air annealing, and c) thermal treatment, air and N2/H2 atmosphere annealing were all irradiated at a laser fluence of 125 mJ/cm2. The films all remained transparent after laser treatment but were found to vary in their conductivity, with b) being the most conductive and c) the least conductive. Films prepared by the deposition of five layers were more poorly suited to laser irradiation than those prepared from three layers, giving less conductive and less transparent films.

Two layer screen printed films (380-31Y screen) prepared by thermal treatment and air annealing were irradiated at a laser fluence of 125 mJ/cm2 and 150 mJ/cm2.

Increased fluence was found to reduce the resistivity whilst maintaining film transparency.

Single layer screen printed films (195T screen) prepared by a) thermal treatment only and b) thermal treatment and air annealing, were irradiated at a laser fluence of mi/cm2 and 175 mJ/cm2. Increased fluence was found to reduce the resistivity in the films pre-treated by thermal treatment and air annealing, whilst all films remained transparent after laser annealing.

Air annealing films appeared beneficial, however the H21N2 annealing of films did not appear to significantly improve the properties of the films.

Pre-treatment Laser Fluence Est. Thickness Appearance Resistivity (mi/cm2) (nm) (acm) Prior to laser 100 Clear 0.42 Thermal Slightly marked, treatment only 125 < 100 Transparent 0.038 Thermal Prior to laser 100 Clear 0.45 treatment and air Marked pattern anneal 125 <100 Transparent 0. 0039 Thermal Prior to laser 100 Clear 0.42 treatment, air and Slightly marked, H2/N2 anneal 125 <100 Transparent 0.032 Table 3: Results for 3 layers, 380-34Y screen Pre-treatment Laser Fluence Est. Thickness Appearance Resistivity (mJ/cm2) (nm) ()cm) Prior to laser 220 Clear 1.6 Thermal Marked Pattern, treatmentonly 125 200 Cloudy 0.012 Table 4: Results for 5 layers, 380-34Y screen Pre-treatment Laser Fluence Est. Thickness Appearance Resistivity (mi/cm2) (rim) (1cm) Prior to laser 140 Clear 0.588 Thermal Marked pattern, treatment and air 125 100 Transparent 0.0059 anneal Marked pattern, 100 Transparent 0.0032 Table 5: Results for 2 layers, 380-31Y screen Pre- treatment Laser Fluence Est. Thickness Appearance Resistivity (mJ/cm2) (nm) (acm) Prior to laser 100 Clear n/a Marked pattern, Thermal ISO <100 Transparent 0.0063 treatment only Marked pattern, <100 Transparent 0.0086 Prior to laser 100 Clear n/a Thermal Marked pattern, treatment and air 150 <100 Transparent 0.0049 anneal Marked pattern, <100 Transparent 0.0034 Table 6: Results for 1 layer, I 95T screen

EXAMPLE 7

Laser Irradiation of Zn41n207 Films Seven layer screen printed films (38034Y screen) prepared by a) thermal treatment only and b) thermal treatment and air annealing were all irradiated at laser fluences of 50, 75, 100, and 150 mJ/cm2. The films showed little visual change on irradiation at low fluence (50 and 75 mJ/cm2) and only small improvements in conductivity. Irradiation at a fluence of 100 mJ/cm2 produced a transparent, conductive film, whilst further increasing the fluence further improved film conductivity but reduced transparency. Better conductivity was achieved on irradiation of five layer screen printed films (380-34Y screen) at 100 mJ/cm2, but such films were starting to cloud at this energy. The three layer films gave similar results.

Two layer screen printed films (380-31 Y screen) prepared by thermal treatment and air annealing were irradiated at a laser fluence of 100 mJ/cm2 and 125 mJ/cm2.

Increased fluence was found to reduce the resistivity whilst maintaining film transparency.

Single layer screen printed films (195T screen) prepared by thermal treatment and air annealing were irradiated at laser fluences of 125, 150 and 175 mJ/cm2.

Increased fluence was found to reduce the resistivity in the films whilst maintaining the transparency of the films.

Air annealing films appeared beneficial to the properties of the films, however the H2/N2 annealing of films did not appear to result in any improvement.

Pre-treatment Laser Fluence Est. Thickness Appearance Resistivity (mJ/cm2) (nm) (Qcm) Prior to laser 300 Clear 26 Thermal 100 250 Clear 0.4 treatment only Marked pattern, 250 Slightly Cloudy 0.034 Prior to laser 280 Clear 8.1 250 Clear 1.13 Thermal 75 250 Clear 0.23 treatment and air Marked pattern, anneal 100 250 Transparent 0.049 Marked pattern, 250 Cloudy 0.02 Table 7: Results for 7 layers, 380-34Y screen Pre- treatment Laser Fluence Est. Thickness Appearance Resistivity (mJ/cm2) (nm) (acm) Prior to laser 220 Clear 6.6 Thermal 75 200 Clear 0. 079 treatment only Marked pattern, 200 Cloudy 0.015 Prior to laser 220 Clear 3.3 Thermal 75 200 Clear 0.042 treatment and air Marked pattern, anneal ioo 200 Cloudy 0.018 Thermal Prior to laser 220 Clear 0.03 9 treatment, air and H2i'N2 anneal ioo 200 Marked Pattern 0.024 Table 8: Results for 5 layers, 380-34Y screen I Pretreatment Laser Fluence Est. Thickness Appearance Resistivi1 (mJ/cm2) (nm) (acm) Prior to laser 100 Clear 5.1 Thermal Marked pattern, treatment only 125 <100 Cloudy 0.014 Prior to laser 100 Clear 2.9 Thermal 75 <100 Clear 0.18 treatment and air Marked pattern, anneal 100 <100 Transparent 0.05 Table 9: Results for 3 layers, 380-34Y screen Pre- treatment Laser Fluence Est. Thickness Appearance Resistivi1 (mJ/cm2) (nm) (acm) Prior to laser 150 Clear n/a Thermal Marked pattern, treatment and air 100 110 Transparent 0.09 anneal Marked pattern, 110 Transparent 0.012 Table 10: Results for 2 layers, 380-31 Y screen Pre- treatment Laser Fluence Est. Thickness Appearance Resistivity (mi/cm2) (nm) (acm) Prior to laser 100 Clear a/a Marked pattern, 0.076 Thermal 125 <100 Transparent treatment and air Marked pattern, 0.01 anneal 150 <100 Transparent Marked pattern, 0.0063 <100 Transparent Table 11: Results for I layer, 1951 screen

EXAMPLE 8

Laser Irradiation of Ga doped ZnO Films Six layer screen printed films (380-34Y screen) prepared by a) thermal treatment only and b) thermal treatment and air annealing were all irradiated at laser fluences ranging from SOto 260 mJ/cm2. The films showed little visual change on irradiation at low fluence (50 and 75mJ/cm2) and only small improvements in conductivity. Films irradiated at fluences = 200 mJ/cm2 were marked with the stitched pattern and were opaque. Surface analysis of these films suggested the formation of a very rough surface.

Irradiation at fluence of 100 or 150 mJ/cm2 produced good quality transparent conductive films. Irradiation of three layer screen printed films (380-34Y screen) at 100 mi/cm2 produced significantly more conductive films, and resistivity was further reduced on increasing the laser fluence to 125 mJ/cm2.

Single layer screen printed films (I 95T screen) prepared by thermal treatment and air annealing were irradiated at laser fluences of 125 and 150 mJ/cm2. Increased fluence reduced the resistivity of the films whilst maintaining their transparency. Air annealing films appeared to be beneficial to the properties of the

films.

Pre-treatment Laser Fluence Est. Thickness Appearance Resistivity (mJ/cm2) (nm) (acm) Prior to laser 300 Clear 13.4 250 Clear 0.48 Thermal Marked pattern, treatmentonly 150 250 Cloudy 0.35 Marked pattern, 250 Opaque 0.19 Prior to laser 280 Clear 19.6 250 Clear 4.6 Thermal ioo 250 Clear 0.11 treatment and air Marked pattern, n/a anneal 200 Rough Cloudy appearance Rough Marked pattern, n/a 260 appearance Opaque Table 12: Results for 6 layers, 380-34Y screen Pre- treatment Laser Fluence Est. Thickness Appearance Resistivity (mJ/cm2) (nm) (Qcm) Thermal Prior to laser 120 Clear 10.6 treatment and air 100 <110 Clear 0.071 anneal 125 <110 Clear 0.04 Table 13: Results for 3 layers, 380-34Y screen Pre-treatment Laser Fluence Est. Thickness Appearance Resistivity (mi/cm2) (nm) (acm) Thermal Prior to laser 100 Clear 18 treatment and air 125 <100 Clear 0.28 anneal 150 <100 Clear 0.14 Table 14: Results for 1 layer, 1 95T screen

Claims (22)

1. A process for coating a substrate with a transparent conducting metal oxide film, which process comprising coating the substrate with a composition comprising a metal carboxylate species and a liquid carrier, heating the coated substrate to evaporate the liquid carrier and form a film of transparent conducting metal oxide and irradiating the film with a laser.

2. A process according to claim 1, wherein the metal carboxylate species comprises one or more species selected from the group consisting of zinc carboxylate species and indium carboxylate species.

3. A process according to claim 2, wherein 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.

4. A process according to claim 2, wherein the indium carboxylate species comprises a species according to formula II: ln(O2CAr)(O2CAr')(O2CAr") (II) wherein the aryl groups are the same or different, substituted or unsubstituted.

5. A process according to any preceding claim, wherein the liquid carrier is a solvent.

6. A process according to any of claims I to 4, wherein the coating composition is an emulsion.

7. A process according to any preceding claim, wherein the coating composition comprises a dopant.

8. A process according to claim 7, wherein the dopant comprises at least one of tin, titanium, vanadium, molybdenum, tungsten, cerium, aluminium, gallium, arsenic, yttrium, fluorine, boron and nitrogen.

9. A process according to any preceding claim, wherein the coating composition comprises one or more additives chosen from the group consisting of surfactants, rheology modifiers, adhesion strengthening additives, drying accelerators and pigments.

10. A process according to any preceding claim, wherein the composition comprises a polymer species.

11. A process according to any preceding claim, wherein the composition is applied to the substrate by spin coating, dip coating, spraying, aerosol, pyrosol, misting, brushing, screen printing or ink-jet printing.

12. A process according to any preceding claim, wherein the substrate is substantially transparent.

13. A process according to any preceding claim, wherein the coated substrate is heated at a temperature between 300 C and 600 C.

14. A process according to any preceding claim, wherein the film is less than 200 nm in thickness, optionally between 100 and 150 nm in thickness

15. A process according to any preceding claim, wherein the coating composition is applied to the substrate, with intermediate drying steps, no more than three times prior to irradiation.

16. A process according to claim 15, wherein the coating composition is applied to the substrate only once prior to laser irradiation.

1 7. A process according to any preceding claim, wherein an additional process step of annealing the substrate in air and/or in a N2/H2 atmosphere is inserted between the step of heating the coated substrate and the step of irradiating the film with a laser.

18. A process according to any preceding claim, wherein the laser irradiating step is carried out at an energy fluence of between 100 and 200 mJ/cm2.

19. A process according to any preceding claim, wherein the transparent conducting oxide exhibits a resistivity of less than 0.4 2cm, optionally less than 0.04 acm.

20. A flat-panel display electrode incorporating a transparent conducting oxide according to any preceding claim.

21. A solar cell incorporating a transparent conducting oxide according to any preceding claim.

22. A process substantially as described herein with reference to the accompanying

Examples.