WO1999035312A1

WO1999035312A1 - Process for producing nanostructured metal oxide thin films on substrates, substrates and films thereof

Info

Publication number: WO1999035312A1
Application number: PCT/SE1999/000014
Authority: WO
Inventors: Lionel Vayssieres; Sten-Eric Lindquist; Anders Hagfeldt
Original assignee: Lionel Vayssieres; Lindquist Sten Eric; Anders Hagfeldt
Priority date: 1998-01-09
Filing date: 1999-01-08
Publication date: 1999-07-15
Also published as: SE9800035D0; AU2083999A

Abstract

A process for producing at least one epitaxial, nanostructured thin film (4) of metal oxide and/or a hydrated metal oxide on at least one substrate (3) placed in a container (1) and a substrate/thin film thereof, said process comprising placing said substrate (3) at any location in said container (1) containing a water solution (2), such that a covering area of said substrate (3) being covered by said solution (2), containing at least one metal salt and additionally can contain other salts, pH-regulator(s), complexant(s), or molecules, whereby from said substrate (3) particles of a metal oxide of the metal from said metal salt is grown, forming said film (4) on said covering area of said substrate (3).

Description

Process for producing nanostructured metal oxide thin films on substrates, substrates and films thereof

Field of the invention The present invention relates to a process for obtaining growth of metal oxide or hydrated metal oxide thin films, in particular epitaxial and nanostructured thin films on substrates.

The present invention is also directed to substrates having such films and thin films produced by the process.

Background of the invention

Thin epitaxial films are generally made by CVD (Chemical Vapour Deposition,

PVD (Physical Vapour Deposition). However, CVD is a high temperature process and is normally performed at a high temperature, for instance above 800 °C, thereby causing troubles if the substrates do not allow such high temperature. They may react or if they are pre-processed wafers (wafers having specific doping profile or sharp interfaces) the pre-processings are destroyed. These techniques result in compact films, exhibiting small surface area. Moreover, flexibility for controlling size and morphology is limited.

Other techniques for producing epitaxial films use low pressure and/or low temperatures instead of high temperatures to form epitaxial thin films on substrates. One technique using low temperature is MBE (Molecular Beam Epitaxy) which provides precise control of stochiometry and thickness of the film. However, the pressure is very low, about 10^"10 Torr. Therefore, this process requires costly vacuum chambers. Normally processes performed at low temperatures would also result in amorphous or polycrystalline materials, not allowing elaboration of metastable or hydrated phases.

Other processes use fullerenes to grow epitaxial thin films (US-A-5 332 723). However also these techniques normally require costly equipment.

US-A-5 332 723 relates to superconducting thin films with fullerenes and a method of making the same. This method requires a vacuum chamber to produce the films. A superconductor cuprate matrix is mixed with fullerenes and is atop a substrate. To produce said superconducting substrate a method is used where a pulsed laser, preferably of excimer type operating at a wavelength of about 248 nanometers focuses its laser beam through a lens and beams through a window of quartz onto a cuprate target inside a vacuum chamber. The cuprate target substrate placed in said chamber receives oxygen from a source and as the laser beams entering said chamber strikes said target, cuprate is deposited onto the substrate positioned vertically above the target. Placed near the target is also fullerenes, as powder and sublimes by the heat inside the vacuum chamber and is thereby delivered to the substrate. The substrate is a single crystal material and is also heated to about 750 °C. Thus, the vapour from the target and the fullerenes are deposited onto the substrate to produce a thin film thereon.

Further techniques to produce thin films are screen printing, flame projection, plasma projection, spray pyrolysis, aerosol deposition, sol gel (spin and dip coating), electrodeposition, MOCVD, cathodic sputtering and thermal evaporation.

In all cases the thin films and in particular the epitaxial thin films are made at high pressures or low pressures and/or high/low temperatures, resulting in a number of drawbacks, which will be discussed in the following. A considerable drawback of these known methods for producing epitaxial thin films is that since they are generally designed to operate at very high or low pressure, they cannot be used to produce high volumes of inexpensive substrates at ambient conditions (25 °C, 1 atm). Additionally, materials that can be used are limited.

A further drawback is their extreme slowness of operation, as they require a long time to produce a metal or a metal oxide film on a substrate.

Yet another drawback is that they usually require more than one step.

Moreover, for certain applications it is also important that the growth creates a porous film onto the substrate and not a compact one, to have a large surface area, which is often not achieved. There is also a need of transparent conductors and/or flexible substrates, for instance to be used in solar cells, or electrochromic devices, ceramics, water purification materials, displays etc. Up till now, no such techniques or substrates have been available at low cost and easy handling.

It is an object of the present invention to overcome the disadvantages brought about with the referenced prior art documents.

Summary of the invention

An object of the invention is to provide a process for producing epitaxial thin metal oxide or hydrated oxide films on substrates, which process does not require high or low pressure and/or costly equipment and is also fast and reproducible. It is also desirable to be able to control the characteristics of the thin metal oxide film applied thereon. This is attained according to the invention through a process as described in claim 1. Surprisingly we have found that by using a water solution containing metal ions it is possible to obtain growth of epitaxial thin metal oxide or hydrated metal oxide films on substrates covered by said solution.

According to a preferred embodiment of the invention a process is provided which accomplishes epitaxial growth of a metal oxide or hydrated oxide thin film on a substrate from a water solution, said film having controlled orientation, morphology, chemical composition, required crystallographic phase and particles of desired size. The process is allowed to be performed under ambient conditions (25 °C, 1 atm). The process can even be applied at low ambient conditions, but preferably at slightly higher temperatures.

A further object of the invention is to provide a process for applying multilayer films of different metal oxides or hydrated metal oxides or morphology, on a substrate. The films can be from a single film to multiple films to composite films. These objects are attained according to the invention through a process for producing multilayer films as described in dependent claims 8-10.

A further object of the invention is to provide substrates having such epitaxial thin films applied thereon. This is attained according to the invention as described in claims 11-16.

Substrates having such films can have applications such as semiconductors, thermal or electric insulators, solar cells and biosensors, solid state electrolytes, supercapacitors, batteries, fuel cells, ceramics, catalysts, electrocatalysts, electrochromic and photoelectrochemical applications such as smart windows and displays. Other applications are for instance anode materials for Li-ion batteries, films as cathode materials for Li batteries, electrochromic devices such as smart windows and displays, catalysts or support for catalysts, magnetic materials, preparation of high-Tc superconductors solid-oxide (SOFC) and solid-polymer fuel- cell (SPFC).

A further object of the invention is to provide thin films that can increase life time of coated components that need fracture resistance, for example.

This is attained according to the invention as described in claims 17-20.

Accordingly, the present invention is directed to epitaxial growth of thin films of metal oxides or hydrated oxides on substrates in a water solution, which rapidly produces the desired film on said substrates. The process is easy to scale up for use in industrial production and it is inexpensive. Moreover, the process has a high degree of flexibility for the elaboration of many materials.

Brief description of the drawings

These and other objects and features of the present invention will be more readily understood from the following detailed description of the preferred embodiments thereof, when considered in conjunction with the drawings, in which like reference numerals indicate identical structures throughout the several views, and wherein:

Fig. 1 is a schematic overall view from the side of a substrate immersed in a water solution in a container.

Fig. 2a is a sectional view of the substrate showed in Fig. 1 having an epitaxial thin film formed thereon, showing a single precipitated film. Fig. 2b shows a substrate as in Fig 2a, but having multiple films formed thereon. Fig. 2c shows a substrate having two layers with particles perpendicular to said substrate.

Fig. 3 is a sectional view of a substrate having a thin film formed thereon, but showing a version using a commercially available conducting substrate. Fig. 4a is a SEM (Scanning Electron Microscopy) photograph of a zinc oxide film on a substrate coated with F-Sn0₂, from above, showing the film grown from the surface of said substrate. Fig. 4b-e are SEM (Scanning Electron Microscopy) photographs of thin films of zinc oxide formed on substrates.

Fig. 5a is a SEM photograph of a film of iron oxide on a substrate having a layer of Sn0₂, after 1 day. Fig. 5b is a SEM photograph after 7 days, with the same conditions as in Fig. 5 a.

Fig. 6a is a SEM photograph a substrate of glass having a layer of F-Sn0₂ and nanoporous Ti0₂ (paint or paste), which is commercially available, having formed thin films thereon, but showing the version used for elaborating iron oxide thereon.

Fig. 6b is a SEM photograph of a substrate having iron oxide with fibres perpendicular.

Fig. 7a is a SEM photograph of a substrate of glass having a layer of F-Sn02 having multiple films of α-Fe₂0₃ on α-Fe₂0₃, having fibres with the same orientation.

Fig. 7b is a SEM photograph of a substrate of glass having a layer of F-Sn02, having multiple films of α-Fe₂0₃ on α-Fe₂0₃, but having fibres with different orientation, one being perpendicular. Fig. 7c is another SEM photograph of the substrate in 7b.

Fig 8 shows the crystallographic structure of α-Fe 0₃ at high resolution (TEM) showing the fine structure of the fibres.

Fig. 9a is a sectional view of a Gratzel solar cell. Fig. 9c and d are photo conversion properties of a solar cell according to the invention. Fig. 9e shows current-voltage characteristics of a hematite photoelectrode as anode in a solar cell Fig. 10 is an enlarged sectional view of a film on a substrate according to the invention, showing different applications according to the invention.

Fig. 11 is a sectional view of a dye-sensitized solar cell according to the invention.

Fig 12 is a sectional view of an intercalation electrode in a battery or other electronic applications according to the invention.

Fig. 13 is a schematic illustration of a viologen/Ti0₂ electrode for displays or smart windows.

Fig. 14 is a schematic illustration of photocatalysis, whereby e.g. pollutants in air or water can be oxidised.

Detailed description of the invention

"Thin films" are in the following films of a thickness lower than 100 μm.

"Epitaxial films" are in the following referred to as films growing from substrates and hence tightly connected thereto in a controlled way.

"Nanostructured films" relate to films made of particles within the nano-range, i. e. 10^"10 - 10^~6 m.

"Hydrated metal oxide" relates to metal hydroxide M(OH)_z and oxohydroxide MO(OH).

In the following "high temperature" is above about 200 °C and "low temperature" is below ambient temperature, i. e. about 25 °C. "High pressure" is above about 1 atm and "low pressure" below. Moreover, "cover" relates to cover, immerse, dip.

"Covering area" is the area of the substrate that is covered by solution.

As defined herein "water solution" means a solution essentially containing water. Water is preferred because of cost, favourable dielectric constant and no toxicity. However, it is also possible to use alcohols mixed with water or other solvents with high dielectric constants.

According to a first preferred embodiment of the process of the invention, as illustrated in Fig 1, a metal oxide film 4 is obtained after having grown directly on a substrate 3 positioned inside a container 1 containing a water solution 2 of metal ions. In this embodiment the metal ions are selected from Fe, Zn, Cr, Al, Ru, Zr, Sn, Ti, Cu, Ni, V, Ce, Nb or any other metal ions from an aqueous metal salt. The solution is also containing some other ions from salts, such as NaN0₃, teframethylammoniumchloride etc or complexants in solution such as amino or acetylacetate or any biological molecule or soluble molecule (organic).

An epitaxial thin film of metal oxide starts to form onto the substrate positioned in the container and after about one day the substrate has a film of sufficient thickness.

In another embodiment of the invention shown in Fig. 2 multiple films of metal oxides are formed onto the substrate. The conditions are the same as for the substrate in Fig. 1, with the exception that different films having different orientation etc. can be made by changing the metal salt and/or other conditions as temperature, pH, precipitation time, concentration. Thereby a substrate having films of different metal oxides, orientations and so on can be achieved. The reaction takes place within a limited space, i. e. in a container, such as a flask, filled with a water solution and preferably closed so that the water solution is not evaporated. The substrate can be at any location in the container, but must be covered by the water solution, containing a metal salt of the preferred metal oxide to be coated onto the substrate. Other salts such as NaCl, or some other salt are added to the solution and if necessary the pH can be adjusted to the proper value, by an acid or a base or a buffer. It is also possible to use complexants, proteins, organic molecules or similar in the solution. The system can be easily and inexpensively recharged by a desired solution containing ions and metal salt of the same kind. Although the formation of metal oxide could take place anywhere in the container, most commonly it will occur at the surface of the substrate, because the surface of the substrate acts as "seeding point" (heteronucleation). Seeding points may be achieved by a pre-treatment of the surface of the substrate. Nevertheless, the walls of the container will normally also be coated with metal oxide and have to be cleaned after the precipitation of the metal oxide on the substrate. When only a limited amount of metal salt is available, all metal salt may be used in the reaction.

The epitaxial growth of the film on the substrate immersed in the water solution, is controlled by the interfacial tension γ between the surface of solid particles of metal oxide and the aqueous solution. The interfacial tension γ is a function of the surface charge σ, which depends on salt in solution and pH. There will also be another surface tension γ_t as the particles precipitate on the substrate. The particles in solution will precipitate, preferably on the substrate and grow to reduce the high surface energy, to become stable. This will result in an epitaxial film tightly connected to the substrate from which the growth started. It is important that the film grows from the substrate. This will result in a very strong connection to the surface of the substrate. Size, morphology, crystallography, porosity and chemical composition are thereby controlled. More specifically, in the case of epitaxial thin films, the need to obtain the right properties of the film growing directly on the substrate is of most importance. Thus, one must be able to control texture of the film, orientation, morphology, crystallographic structure, chemical composition and size of particles in the film. Furthermore, the growth of particles from the substrate can be like fibres extending up from the surface or parallel to the surface, or any desired direction. The fibres growing from the substrate can consist of bunches of fibres oriented on the substrate for instance to get the right properties. The fibres have very small diameters and can be perpendicular or parallel or some other angle between 10-90 ° to the substrate and of a length of up to 100 micrometers.

Furthermore hollow fibres (tubes) may also be obtained in certain conditions.

The properties of the grown film can be controlled by parameters in the water solution, such as pH, temperature, stirring, i. e. convection, kinetics, i. e. nucleation time and ageing, chemicals, i. e. metal salt.

The concentration of active metal ions in the solution is up to saturation of the solution. The temperature can be up to about 150 °C.

The thickness of the precipitated layer will vary up to several μ .

The precipitation time will vary from a few minutes to several days, say seven days, at a temperature up to required, depending on which metal is to be coated on which substrate. The time depends on thickness of the film, size and crystallography of the particles and can be several days or even only one minute or even less. Furthermore, metal-ion transport in water solution involves solvent-ligand exchange, therefore co-ordination chemistry is of most importance. This is relevant to the reactivity of the ions in solutions at the substrate/electrolyte interface. However, this is not further discussed. All materials must have mechanical, chemical and electrochemical stability. Surface electrochemical properties of oxide surfaces are intimately related to structural properties.

When the precipitation is repeatedly turned on and off by having different baths of solutions, multilayer thin films will be achieved, having different properties depending on the different solutions and thereto accompanying parameters. It is also possible to have a first layer having one orientation, followed of a second layer having a different orientation or the same with different size of particles or morphology. Also multiple layers of the same metal can be build up, to the desired thickness in order to increase the surface area or change the optical properties of one or more of the layers or to match the optical properties of the assembled layers. The film(s) are hard and cannot be scratched easily. Applications are for instance optoelectronics e. g. information displays, variable reflectance and/or emittance mirrors, smart windows or batteries, multicoatings in optics etc.

Thus, the film growth is accomplished by enhanced control of the chemical reaction of precipitation (nucleation and growth) and the kinetics and thermodynamics of the reaction. Specifically, the films are epitaxial, the size range, the morphology, the chemical composition, the orientation, the polydispersity of the particles as well as the crystallographic phase, i. e. allotropy of the films are controlled. Therefore, it is now possible to tune the films in order to optimise their physical and chemical properties, such as electrical, optical, magnetic, catalytic properties etc. It is also possible to obtain metastable crystallographic phase, which is normally hard to obtain. The metastable phase may then be heat-treated to obtain the stable one.

Furthermore, the present invention makes it possible to produce purpose-built high tech materials, such as nanostructured and composite thin films for several purposes and applications in thin film technology, e. g. solar cells, displays, batteries etc. In particular nanostructured metal oxide films have use as anode or cathode materials for Li-ion batteries, smart windows, information storage displays, catalysts or support for catalysts, preparation of high-Tc superconductors, solid-oxide (SOFC), solid-polymer fuel-cells (SPFC) and magnetic properties. The films can be subject to posttreatment and processing such as laser etching, photoelectrochemical etching, depending on the application they will be used for.

The advantages of the invention over prior art include lower cost, greater simplicity, less volume and include substantially increased rate of producing epitaxial metal oxide films on substrates and to obtain films having specific controlled characteristics on the substrates. Also as compared to almost any system there is much greater safety, because of non-poisonous solvents and normal pressure and/or temperatures used. Moreover, the cost of precursor and chemicals are reduced.

While the described embodiments of this invention constitutes single layer substrate it is fully within the scope of this invention to use substrates of composite type and/or pre-treated with coatings, for instance conducting coatings, which would work as well as a single layer substrate.

In another embodiment of the invention shown in Fig. 3, a substrate provided with a conductive surface may be selected from the many commercially available grades of such substrates. A particularly desirable product is a transparent substrate. The conducting coating can have almost any thickness. Other coatings having special properties and the like may be substituted for the conducting coating. Except for this conductive surface on the substrate, the conditions are the same as in Fig. 1 or Fig. 2.

Fig. 10 shows a similar film as in Fig. 3 a, but better illustrating a film C according to present invention; transparent or non-transparent depending on application. Here the film is depicted as an ordered assembly of columnar crystals extending out from the surface.

This embodiment will be further described in the following.

Effects of different temperatures, concentrations, time and pH will be further described in the following specific examples.

Quite surprisingly, the substrate can also be almost any coating free substrate. The substrate can be made of glass, tempered or untempered, or plastics (FEP,PP...), ceramics, metals etc. The thickness of the substrate may vary almost independently, the major consideration being to keep the thickness to a dimension which is suitable for the application. Slight imperfections of the surface of the substrate merely increases the growth effect.

The invention is not limited to the use of a single substrate in the container, but several of several kinds can be positioned in the container simultaneously and treated at the same time to increase production (scale up).

Other types of films of metal oxides could be developed in accordance with this invention by using the appropriate material and by providing minor variations to the operating parameters. Hydroxide or oxohydroxide metal films, M(OH)_z and MOOH respectively, can be transformed to more stable oxides by heat treatment afterwards.

The invention will now be further illustrated by the following examples. The examples are only given to illustrate the invention and should not be considered as limiting. In the examples all concentrations are molar and all measurements are in the metric system, unless otherwise stated.

Examples Example 1

Layer of zinc oxide A substrate of conducting glass was placed in a 500 ml glass flask (Pyrex) and a water solution having the following composition was filled in the container, so that the substrate at the lower part of said container was totally immersed in the solution. Films of ZnO soon precipitated and increased to a thickness of 0,6 μm (one layer) after one day at 70 °C. The pH was about 6. ZnO has a crystallographic structure called wurtzite even below 100 °C, which is a considerable advantage.

0,65 g Zn(NO₃)₂ (Fig. 4a)

0,3 g HHT (metamin) 400 ml water

The area of the substrate is approximately 10 X 2 cm 2

Fig. 4a is a SEM (Scanning Electron Microscopy) photograph of the film of zinc oxide, showing that the fibres are growing from the surface of the substrate. This photograph clearly shows the structure of the film of zinc oxide, having fibres of small diameter, extending from said surface. Fig. 4b is another photograph of a film of zinc oxide on a similar substrate. Also this photograph shows how the particles extend from the surface of the substrate.

Effect of temperature and/or time

Fig. 4c shows the same as in Fig. 4a, but using increased time and/or increased temperature. The amount as well as the length of the fibres have increased compared to Fig. 4a, illustrating the effect of time and/or temperature on the film as obtained. Fig. 4 d shows the effect of time and concentration, as they increase (7 days at 70 °C). The amount of particles are very large, but the structure of the film is still porous.

If the ageing time is very long, say 7 days, the morphology will change. This effect is shown in Fig. 5d. The particles in the film on the substrate are singles and look like spheres (Note: This Fig 5d. shows iron oxide, which is not heat-treated).

Example 2

Hydrated metal oxide

A substrate of conducting glass was placed in a container (same glass flask as in example 1) and a water solution having the following composition was placed in the container, so that the substrate was totally immersed in the lower part of said container. A film of β-FeOOH soon precipitated and increased in thickness to 0,6 μ (1 layer) after one day at 100 °C. The pH was adjusted to 1,5 by adding HC1.

FeCl₃0,15 M NaN0₃ 1,0 M Adjust pH to 1,5

The film of β-FeOOH was heat-treated at 500 ° C and transferred to α-Fe₂0₃ (hematite).

The obtained surface (film) after a scratch with a blade is shown in Fig. 5 a, which clearly shows that an oriented bunch of hematite fibres perpendicular to the surface of the substrate was obtained.

Example 3 Effect of pH

Same conditions as in example 1, but higher pH.

This example (not illustrated) shows that pH has effect on the thickness of the film. A slight higher pH will decrease the thickness, which is depending on the chemistry which is thereby changed. This will increase the surface charge, thereby decreasing the growth as the particles will aggregate to decrease the area.

Example 4

Effect of morphology

Fig. 5b shows the same conditions as in example 2, but 7 days ageing time, instead of one day. This affects the morphology, the particles are spherical. It is evident that the films are more compact and have increased thickness.

Example 5

Zinc and iron oxide growth on commercially available F-SnO? glass substrate A substrate of glass having a layer F-Sn0₂ and nanoporous Ti0₂ (paint or paste) coated thereon (all of them commercially available) was placed in the container and a water solution having the following components was filled in the container, so that the substrate at the bottom of said container was totally immersed. Films of ZnO oxide precipitated and increased to a thickness of 2,0 μm (one layer) after one day at 100 °C. The pH was 6. See Fig. 6a.

Zn(N0₃)₂ + complexant (aminocomplex)

Fig. 6b shows iron oxide on a substrate coated with F-Sn0₂, whereby the particles in the film have perpendicular orientation to the substrate. Example 6

A substrate of glass having a layer F-Sn0₂ and nanoporous Ti0₂ (paint or paste) coated thereon (all of them commercially available) was placed in the container (as in previous examples) and a water solution having the following concentrations was filled in the container, so that the substrate at the bottom of said container was totally immersed. Films of β-FeOOH precipitated and increased to a thickness of 0,6 μ (1 layer) after 1 day at 100 °C. The pH was 6. The β-FeOOH was then heat- treated to more stable α-Fe₂0₃. See Fig. 7a showing films with fibres having the same orientation and Fig. It is evident that the layers are more compact and have increased thickness. Fig. 2c shows how these fibres can be parallel arranged to each other, connecting both films. 7b shows layers with fibres having different orientation (perpendicular), which improves the optical properties (right region).

Furthermore, complexants of metal salts, such as or phosphorous ions with amines can be used. The pH-value of the solution can be adjusted by any base or acid, and ionic strength (concentration of salt).

From the foregoing examples it is apparent that the process for producing epitaxial layers of different types according to the invention is particularly advantageous compared with conventional techniques and in particular as substrates are not destroyed at high temperatures and the simplified and good control of the desired properties of the films.

In the following a number of applications will be described in more detail.

Photovoltaic and solar cells A conventional solid state cell is a semiconductor device which on illumination can produce electrical energy. It can consequently be used as solar cell. There are numerous other applications in minicomputers, watches, shelf labels etc. In the heart of the design of a photovoltaic cell is a junction in which an electrical field spontaneously is form due to choice of materials. The material can be the same through out the junction only differing in choice of dopant. E.g. single crystals of silicon can be doped with aluminium and phosphorous creating a p- type respectively n-type semiconducting silicon. Materials of p- and n-type in and electrical contact forms a p-n junction.

If the junction creating the electrical field in the semiconductor is created at a) the interface of a semiconductor and metal, or b) the interface of a semiconductor and liquid electrolyte, the junction is called a Schottky junction.

On illumination - independent of type of junction - most of the charge separation normally takes place in the in the junction region. Diffusion of charge carrier from regions outside the junction may however contribute to the current. All the photovoltaic cells described above are based on single crystal- or polycrystalline solid homogeneous semiconductors. Those containing polycrystalline materials are in the category of thin photovoltaic cells. Typical semiconductors in such cells are amorphous silicon, CdSe, CuInSe₂. Cells containing a liquid electrolyte are often called photoelecfrochemical cells (PEC).

There is a new type of photovoltaic cells based on nanoporous - nanocrystalline, often called nanostructured, photovoltaic cells. It is a PEC, because it contains an electrolyte in which a reversible redox system is dissolved. The cell is a sandwich structure. It is characterised by a porous film of a large band gap semiconductor (or insulator) on a conducting, often transparent substrate forming a working electrode (WE). Sandwiched on top of the WE is a counter electrode (CE), which can be a metal sheet or a transparent conducting oxide film on a glass, depending on the design of the cell and incidence of light. The electrolyte is placed in-between WE and CE and in the pores of the nanostructured WE. Thus it penetrates the cell from the rear contact of the WE to the CE. The cell is often, but need not necessarily be dye-sensitised. When a nanostructured large band gap semiconductor is used, the WE absorbs light only in the UV region and a dye adsorbed on the nanostructured electrode then extends the absorption of light into the visible region. This makes the cell more efficient as solar cell. The solar cell efficiency of the so called Gratzel cell, which is a dye-sensitised nanostructured solar cell, is claimed to be 11%. The WE in the Gratzel cell is made of Ti0₂. The dye is typically a Ruthenium bipyridyl complex and the electrolyte is the Iodide/three Iodide redox couple in acetonitrile or other aprotic solvent. The iodide/three iodide works as a shuttle for electrons between the dye molecules on the nanostructure and the CE. An advantage with the dye-sensitised photovoltaic cells is that they can be made more or less semitransparent and in different colours. Thus they can be mounted as windows in buildings and different devices such as watches etc.

In use, several solar cells are coupled together, thereby forming a solar panel, having the desired voltage. The cells are normally protected from the surroundings by some coating, for instance glass. The efficiency, i. e., generated solar energy/incident light depends on the semi-conductor. A typical value is about 10-15 %. Advanced and normally very expensive cells can reach above 20 % (measured at 1000 W/m = 1 SUN intensity of incident light). However, they have severe disadvantages, in particular they are expensive. Moreover, they cannot be used for windows or indoor transparent applications, as they are not transparent.

Many solar cells of prior art need to be fired, e. g. the above mentioned Gratzel cell. Therefore it is not possible to use substrates of polymeric materials, which can be flexible, because they are easily destroyed by the heat during manufacturing. The Gratzel-types are made of a number of interconnected metal oxide particles in the form of more or less spherical crystallites or nano crystals in contact with and on a conducting surface as showed in Fig. 9a (Only two layers are showed, but can be up to several hundreds). To work properly, the particles must be very small, approximately about 20 nm in diameter, to be impregnated with a sufficient amount of a special chromofor dye. The manufacturing of such a solar cell requires several steps, including firing at high temperature to contact the spheres to each other and expensive precursors. If not, no current will be available. The redox electrolyte is iodide/three iodide, which is highly corrosive. The cell is sensitive to 0₂ and H₂0. The cell is also difficult to seal and stability problems may occur because of the dye. For example, the dye used today desorbs from the metal oxide surface if water is present in the electrolyte.

The process for producing epitaxial thin film as described according to the invention may solve part of these problems, having substrates which can be flexible and made of plastic material, as schematically showed in Fig. 10. Instead of the many more or less spherical particles, fibres can be used, which has single crystals arranged in a line preferably perpendicular to the substrate inside it, and connected to the substrate, without using firing. One cell comprises many such fibres, arranged in bunches. As the fibres are also strongly connected to the surface, the contact is very good, resulting in a low energy barrier. The substrate is preferably made of transparent material, such as glass or, plastic provided with a conducting transparent film e. g. fluorine doped tin oxide or indium tin oxide. The film may be made of a transparent oxide e. g. ZnO and dye-sensitised or e. g. iron oxide. One example of an electrode based on fibres is a F-doped Sn0₂ substrate with perpendicular iron oxide fibres thereon, giving favourable optical absorbance, making the use of a dye unnecessary. The efficiency is very much improved compared to a system with spherical iron oxide particles and the manufacturing only requires a single step.

The new iron oxide cell is red-brown and does not require any dye, to be able to absorb visible light. This is better described in accompanying drawings 9c-d, showing action spectra of the cell. Action spectra define how many percent of photons give rise to electrons. Compared to prior cells based on spherical iron oxide particles, the efficiency has increased very much. Moreover, the iron oxide cell is not sensitive to water or 0₂ and hence have chemical stability, which is clearly indicated in Fig. 9c-d. Thus, the cell is simple, stable and inexpensive to manufacture.

Typical values for one cell according to the invention are 0,4 mA and 0,4 V (1 cm3) at 0,3 Sun light intensity. The overall efficiency is about 0, 1 % for a non-optimised cell.

This cell is only about 10 % of the thickness of the traditional dye/Ti0₂ Gratzel cell and will of course be very much improved if the thickness is increased.

In use many solar cells are connected to each other in parallel or in series, for instance by using a grid. This can be easily accomplished according to the invention, by using some laser scribing blade scratch technique, creating a grid-pattern on the substrate.

Sensors

A sensor is defined as a device that can interact with species, and converts the interaction into an observable signal. Thus, the sensor has to be able to interact with the species of interest, hereinafter called the analyte. Also other species are normally present which can cause the sensor to create a signal, despite they are not wanted, and therefore can obscure the desired signal. Such species are called interferents, and because of these the sensor has to be able to distinguish between signals arising from interferants and the desired analyte. This can for instance be solved by using biological molecules such as enzymes or other types of biological material such as tissues, which can accomplish for both a selectivity mechanism and the desired interaction with the analyte, thereby giving rise to a measurable signal. These kind of sensors are referred to as "biosensors" . In prior art a type of sensor used is the "enzyme based redox polymer wired amperometric sensor". This sensor uses enzymes and redox polymers based on osmium complexes. The osmium compounds are generally regarded as toxic.

Furthermore, other types of sensors such as gas sensors and chemical sensors, which change the conductivity of the film when gas or chemicals are adsorbed and thereby release an electron can be made according to the invention. One well-known gas sensor is zinc oxide, and also hematite has been reported as gas sensor. Other types that can be made are e. g. sensors comprising proteins, anti-bodies, viruses etc. All these sensors can be achieved according to the invention. The sensors made according to the invention are porous, which is a great advantage, as a porous surface can adsorb much more enzyme compared to a more compact surface.

A schematic drawing of an electrode or an optical device according to the invention which can be used in several different kind of applications depending on the materials is illustrated in Fig. 10.

A is the substrate. B is the conducting film layer. C is the metal oxide film.

D is an electroactive component at the surface of the film, showing a version of film on a substrate which can be used as a sensor.

E is a solution with redoxactive components in an electrolyte, a gas or a gas mixture.

The components are variable depending on the desired application.

Different embodiments will be described in detail as follows: 1) If D is a dye able to inject an electron into the conducting band of C, or deliver a hole to the valence band of C, on absorption of a photon, the arrangement constitutes the working electrode (WE) of a photovoltaic cell.

2) If D is redox active organic, or inorganic, compound able to electrically interact with C and D changes colour after accepting or loosing, an electron, the film is an active component in a display.

3) If D is redox active molecule which electrically interact with C directly or via a mediator and the redox active molecule interacts specifically with a certain component in the electrolyte, the film is an active part of an amperometric sensor.

4) If D is a biomolecule, e. g. a cytocrome or redox enzyme which electrically interact with C directly or via a mediator and the biomolecule interacts specifically with a certain chemical component in the electrolyte, the film is an active part of an amperometric biosensor.

5) If D is not present and a chemical component e. g. Li+ or H+ is intercalated into C directly, the film is an active component in a battery or if on intercalation, the colour of the film changes, the film is an active component in a display or smart window.

6) If the colour of D changes on specific interaction, without or with electron transfer from D to C, with a component in a solution, as under 3-5 or in a gas and any colour change is related to concentration of component(s) in the gas or solution, then the film is an active component in an optical sensor.

Other areas can be photocatalysis, water purification, magnetism, fuel cells, nonlinear optics, protective coating, electroluminescence, photoluminescence superconductors and proton conductors. Fig. 11 shows a cross section of a two electrode system, a dye-sensitised solar cell according to one embodiment of the invention.

A shows the nanostructured electrode and the counter electrodes before assembling. B shows assembled electrodes with dye on the nanostructured film. C shows the charge transport that follows on the photo excitation of the dye molecule.

Fig. 12 shows an electrode in an intercalation battery, whereby on intercalation charge is stored in the metal oxide film and the nanostructured film turns deep blue.

Fig. 13 shows a viologeπ/Ti0₂ electrode for use in displays or smart windows, creating sharp contrast, different colours, fast switching times etc.

Fig. 14 shows photocatalysis which can be used for self cleaning of windows or as dutch tile for desinfection or cleaning. The pollutant in the air or in a solution will be oxidised.

Claims

1. A process for producing at least one epitaxial thin film (4) of metal oxide and/or hydrated metal oxide on at least one substrate (3) placed in a container (1), comprising placing said substrate (3) at any location in said container (1) containing a water solution or a mixture thereof (2), such that a covering area of said substrate (3) being covered by said solution (2), containing at least one metal salt, which solution (2) additionally can contain other salts, pH-regulator(s), complexant(s), dopants or molecules, whereby from said substrate (3) particles, such as fibres of a metal oxide of the metal from said metal salt is grown, forming said film (4) on said covering area of said substrate (3).

2. A process according to claim 1, wherein the metal salts are selected from metal salts soluble in water.

3. A process according to any of the claims 1-2, wherein the pH in the water solution (2) is from 0-14.

4. A process according to any one of the claims 1-3, wherein the temperature of the solution (2) is above 0 ┬░C, preferably above ambient temperature, up to 150 ┬░C.

5. A process according to any one of the claims 1-4, wherein the time for growing of the film (4) on the substrate (3) is from about one minute up to 8 days.

6. A process according to any one of the claims 1-5, wherein the particles, such as fibres in said film (4) is grown substantially perpendicular orientation or some other angle in the range 10-90 ┬░ to said substrate (3).

7. A process according to any one of the claims 1-5, wherein the particles, such as fibres in said film (4) is grown substantially parallel orientation to said substrate (3).

8. A process according to any one of the claims 1-7, wherein a first film (4) of one metal oxide is formed in a first step on the substrate (3) and a second film (4) of the same or another metal oxide and/or morphology and/or particle size is formed on said first film (4), by using a different solution (2), comprising said second metal in said second step.

9. A process according to claim 8, wherein the particles in said second film (4) is grown in different orientation, than the particles in said first film (4).

10. A process according to claim 8 or 9, wherein said second film (4) is provided with other film(s) (4).

11. A subsfrate having at least one film (4) applied thereon, which film is produced by the process according to any one of the claims 1-10.

12. A substrate (3) according to claim 11 for use as a solar cell, wherein the oriented particles in said film (4) have a photoactive component attached thereon, the fibres being arranged substantially perpendicular to said substrate (3), wherein the substrate (3) is transparent having fluorine doped tin oxide thereon, and fired at a raised temperature.

13. A subsfrate (3) according to claim 12, wherein said photoactive component is a dye.

14. A substrate (3) according to claim 11 for use as a biosensor having interaction with an analyte to be measured, wherein said film (4) is further provided with biological molecules (5) or other types of biological material, such as tissues, which can accomplish for both a selectivity mechanism and the desired interaction with the analyte, thereby giving rise to a measurable signal, by adhering said biological molecules or material (5) by adsorption.

15. A substrate (3) according to claim 11 for use in a photovoltaic cell, wherein the first film (4) of oriented fibres on said substrate (3) made of glass, having a conducting coating applied thereon, comprises iron oxide (hematite ╬▒-Fe₂0₃) or iron oxide.

16. A substrate (3) according to claim 11 for use as a sensor such as an optical, gas or chemical sensor, wherein redoxactive molecules which electrically interact with the fibres or via a mediator and specifically interacts a certain chemical component in an electrolyte are provided onto the fibres in the film (4).

17. A metal oxide thin film for use in batteries, which film is produced by the process according to any one of the claims 1-7, wherein an electroactive component is not present and a chemical component e. g. Li+ or H+ is intercalated directly into the metal oxide.

18. A metal oxide thin semiconductor film for electrochromic applications, such as displays or smart windows, which film is produced by the process according to any one of the claims 1-7, wherein an electrochromic component, such as viologen, amines, organometallic compounds is present and/or a chemical component is intercalated directly into the metal oxide, whereby the colour of the film (4) changes.

19. A film produced by the process according to any one of the claims 1-7, wherein the film (4) has magnetic properties.

20. A film for gas or chemical sensors, which film is produced by the process according to any one of the claims 1-7, wherein the film (4) comprises ZnO microtubes.