Abstract:
An electrochemical cell and a method of manufacturing the same are provided. The electrochemical cell comprising: a first conductive layer; a metal oxide layer formed on the first conductive layer, the metal oxide layer comprising a plurality of adjacent metal oxide cells, spaced from one another; a functional dye layer formed on the metal oxide layer; a second conductive layer; and an electrolyte between the functional dye layer and the second conductive layer, wherein at least one of the first and second conductive layers is transparent, and wherein the metal oxide layer is formed from a metal oxide particle dispersion liquid.

Description:
This is a Continuation of application Ser. No. 11/598,723 filed Nov. 14, 2006. The disclosure of the prior application is hereby incorporate by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an electrochemical cell structure and a method of fabrication. In particular, the present invention relates to a metal oxide particles dispersion liquid used in the formation of a metal oxide layer. 
     BACKGROUND OF THE INVENTION 
     The International Energy Agency&#39;s “World Energy Outlook” predicts that global primary energy demand will increase by 1.7% per year from 2000 to 2030. It also predicts that 90% of this demand will be met by fossil fuels. Consequently, there will be a 1.8% per year increase in carbon dioxide from 2000 to 2030, reaching 38 billion tonnes in 2030. Cleaner, renewable energy sources, including solar cells, have long been heralded as counters to this increased pollution trend. While advanced silicon based solar cells are now widely commercially available, their uptake has been slow due to high production costs, a lack of robustness and associated visual pollution resulting from the large surface exposure requirements. 
     Dye Sensitised Solar Cells (DSSC) are an alternative to crystalline solar cells that are cheaper than crystalline solar cells to produce. However, DSSC&#39;s are less efficient than crystalline solar cells. Therefore, DSSC&#39;s require significant area coverage to be effective power generators. 
     U.S. Pat. No. 4,927,721 entitled “Photo-Electrochemical Cell”, by M Gratzel et al. discloses a typical DSSC. As illustrated in  FIG. 1 , the DSSC  10  comprises a first transparent insulating layer  1 ; a first transparent conductive oxide (TCO) electrode layer  2 ; a transparent metal oxide layer  3  of titanium dioxide (TiO 2 ); a molecular monolayer of sensitiser (dye)  4 ; an electrolyte layer  5 ; a second transparent conductive oxide (TCO) electrode layer  6 ; and a second transparent insulating layer  7 . 
     A DSSC generates charge by the direct absorption of visible light. Since most metal oxides absorb light predominantly in the ultra-violet region of the electromagnetic spectrum, a sensitiser (dye)  4  is absorbed onto the surface of metal oxide layer  3  to extend the light absorption range of the solar cell into the visible light region. 
     In order to increase the amount of light that the metal oxide layer  3  and the sensitiser (dye) layer  4  can absorb, at least some portion of the metal oxide layer  3  is made porous, increasing the surface area of the metal oxide layer  3 . This increased surface area can support an increased quantity of sensitiser (dye)  4  resulting in increased light absorption and improving the energy conversion efficiency of the DSSC to more than 10%. 
     An electrochromic display (ECD) is a new type of display, which has a unique property of bi-stability. 
     An electrochromic display (ECD) is a relatively new electrochemical, bi-stable display. While the application is different to the DSSC, these devices share many physical attributes, illustrated in  FIG. 1 , exchanging the sensitiser (dye) layer  4  by an electrochromic material layer which undergoes a reversible colour change when an electric current or voltage is applied across the device; being transparent in the oxidised state and coloured in the reduced state. 
     When a sufficient negative potential is applied to the first transparent conductive oxide (TCO) electrode layer  2 , whilst the second transparent conductive electrode oxide (TCO) layer  6  is held at ground potential, electrons are injected into the conduction band of the metal oxide semiconductor layer  3  and reduce the adsorbed molecules (the coloration process). The reverse process occurs when a positive potential is applied to the first transparent conductive oxide (TCO) electrode layer  2  and the molecules become bleached (transparent). 
     A single electrochromic molecular monolayer on a planar substrate would not absorb sufficient light to provide a strong colour contrast between the bleached and unbleached states. Therefore a highly porous, large surface area, nanocrystalline metal oxide layer  3  is used to promote light absorption in the unbleached state by providing a larger effective surface area for the electrochromophore to bind onto. As light passes through the thick metal oxide layer  3 , it crosses several hundreds of monolayers of molecules coloured by the sensitiser (dye)  4 , giving strong absorption. 
     Since the structure of both electrochemical devices is similar, we describe only the method of DSSC manufacture as an example. Equally, this process could be applied with little modification to the ECD manufacture. 
     In order to manufacture the DSSC  10  illustrated in  FIG. 1 , a metal oxide layer  3  of several microns thickness is deposited onto the first transparent conductive oxide (TCO) electrode layer  2 , using any one of several techniques, such as screen printing, doctor blading, sputtering or spray coating a high viscosity paste. A typical paste commonly consists of water or organic solvent based metal oxide nanoparticle suspensions (5-500 nm diameter), typically titanium dioxide (TiO 2 ), a viscosity modifying binder, such as polyethylene glycol (PEG), and a surfactant, such as TRITON-X. Following deposition the paste is dried, to remove the solvent, and then sintered at temperatures up to 450° C. This high temperature process modifies the metal oxide particle size and density, and ensures the removal of the organic binder constituents, such as polyethylene glycol (PEG) to provide a good conductive path throughout and a well defined material porosity. Sintering also provides good electrical contact between the metal oxide particles  3  and the first transparent conductive oxide (TCO) electrode layer  2 . 
     After drying and cooling, the porous metal oxide layer  3  is coated with sensitiser (dye)  4  by immersion in a low concentration 1 mM) sensitiser (dye) solution for an extended period, typically 24 hours, to allow absorption of the sensitiser (dye)  4  onto the metal oxide layer  3  through a functional ligand structure that often comprises a carboxylic acid derivative. Typical solvents used in this process are acetonitrile or ethanol, since aqueous solutions would inhibit the absorption of the sensitiser (dye)  4  onto the surface of the metal oxide layer  3 . 
     The first transparent conductive oxide (TCO) electrode layer  2 , having the porous metal oxide layer  3  and sensitiser (dye) layer  4  formed thereon, is then assembled with the second transparent conductive oxide (TCO) electrode layer  6 . Both electrode layers  2 ,  6  are sandwiched together with a perimeter spacer dielectric encapsulant to create an electrode-to-electrode gap of at least 10 μm, before filling with the electrolyte layer  5 . The spacer material is most commonly a thermoplastic that provides an encapsulation seal. Once the electrolyte layer  5 , which is most commonly an iodide/triiodide salt in organic solvent, is introduced, the DSSC is completed by sealing any remaining aperture with either a thermoplastic gasket, epoxy resin or a UV-curable resin to prevent the ingress of water and hence device degradation. 
     Most, if not all, of the materials used to fabricate the DSSC can be handled in air and also under atmospheric pressure conditions, removing the necessity for expensive vacuum processes associated with crystalline solar cell fabrication. As a result, a DSSC can be manufactured at a lower cost than a crystalline solar cell. 
     The ECD fabrication process is very similar to that for the DSSC, with several exceptions. The porous metal oxide layer  3  is often patterned by screen printing to provide a desired electrode image, allowing the device to convey information by colouring or bleaching selected regions. Additionally, the sensitiser (dye) layer  4  is replaced with an absorbed electrochromophore material layer. Furthermore, a permeable diffuse reflector layer, typically large particles of sintered metal oxide, can be positioned between the first and second electrode layers  2 ,  6  to increase the viewed image contrast. 
     U.S. Pat. No. 5,830,597, entitled “Method and Equipment for Producing a Photochemical Cell”, by H Hoffmann also discloses a DSSC  100 . As illustrated in  FIG. 2 , the DSSC  100  comprises a first substrate  101  of glass or plastic; a first transparent conductive oxide (TCO) layer  102 ; a titanium dioxide (TiO 2 ) layer  103 , a dye layer  104 ; an electrolyte layer  105 ; a second transparent conductive oxide (TCO) layer  106 ; a second substrate  107  of glass or plastic; and insulating webs  108 ,  109 . The insulating webs  108 ,  109  are used to form individual cells  110  in the DSSC  100 . 
     An individual cell  110  formed between the insulating web  108  and the insulating web  109  is different from the adjoining individual cell  110  formed between the insulating web  109  and the insulating web  108 . This is because the TiO 2  layer  103  and the electrolyte layer  105  are interchanged in each adjoining individual cell  110 . Thus, the electrical polarity of the adjoining individual cells  110  is opposite. This alternate division of different layers results in the formation of conducting layers  111  from the electrically conductive layers  102  and  106 , each conducting layer  111  connecting a positive (negative) pole of one individual cell  110  to the negative (positive) pole of an adjacent individual cell  110 . The resultant structure provides a method of increasing the overall DSSC output voltage, without the necessity of incorporating a multi-layered structure. 
     In order to improve the incident photon to current conversion efficiency and control the stability/reproducibility of the DSSC performance, it is important to precisely control the physical properties of the metal oxide layer, and hence the absorption of the sensitiser (dye) molecule. However, metal oxide layer fabrication using screen-printing often results in a ±5% film thickness variation caused by residual blocked or dirty screen cells, adhesion to the screen during separation from the substrate surface and trapped bubble expansion during drying, caused by the inability to completely outgas a viscous paste. Other methods, such as doctor-blading, also suffer from an inability to provide a well defined thick metal oxide layer without significant spatial deviations. Subsequent porosity and film quality deviations are therefore likely to occur throughout such metal oxide layers, resulting in a degradation of efficiency and image quality for the DSSC and ECD, respectively. 
     In the case of the ECD, screen-printing demands are further exacerbated by the requirement to create ever finer metal oxide layer features for higher quality images, i.e. increase the dots-per-inch (dpi) for a pixelated display. As the dpi increases, the smallest feature size becomes limited as the screen mesh size approaches the mesh partition width. 
     As a result, fabrication of an electrochemical device based on a functionally sensitised thick porous metal oxide layer, as for the DSSC and ECD, using the aforementioned fabrication techniques are inappropriate from the view points of device reproducibility and adaptability to large size device production. 
     SUMMARY OF THE INVENTION 
     The present invention aims to address the above mentioned problems of manufacturing electrochemical cells (DSSC&#39;s and ECD&#39;s) of the prior art, to improve the efficiency with which they are made and thus further decrease their fabrication costs. 
     In a first embodiment of the present invention an electrochemical cell is provided. The electrochemical cell comprising: a first conductive layer; a metal oxide layer formed on the first conductive layer, the metal oxide layer comprising a plurality of adjacent metal oxide cells, spaced from one another; a functional dye layer formed on the metal oxide layer; a second conductive layer; and an electrolyte between the functional dye layer and the second conductive layer, wherein at least one of the first and second conductive layers is transparent, and wherein the metal oxide layer is formed from a metal oxide particle dispersion liquid. 
     In one embodiment the metal oxide particle dispersion liquid is a water based liquid. In another embodiment the metal oxide particle dispersion liquid is an alcohol based liquid. In another embodiment the metal oxide particle dispersion liquid comprises a plurality of metal oxide particles having a diameter of between substantially 5 nm to 500 nm. In another embodiment the plurality of metal oxide particles have a diameter of substantially 18 nm. 
     In a further embodiment the metal oxide particle dispersion liquid further comprises a viscosity modifier. In another embodiment the viscosity modifier is polyethylene glycol or polyethylene oxide. In another embodiment the viscosity modifier has a concentration less than 5% w/w. In another embodiment the metal oxide particle dispersion liquid further comprises a binder. In another embodiment the binder is polyethylene glycol or polyethylene oxide. In another embodiment the binder has a concentration less than 5% w/w. In another embodiment the metal oxide particle dispersion liquid further comprises a surfactant to adjust surface tension. In another embodiment the surfactant is Triton-X. 
     In one embodiment the electrochemical cell further comprises: separating means formed on the first conductive layer and surrounding each of the plurality of adjacent metal oxide cells. In another embodiment the separating means is a polymer pattern or a polyimide pattern. In another embodiment at least part of the separating means is hydro- and/or oleophobic and wherein the first conductive layer is hydro- and/or oleophilic. In another embodiment the separating means forms a matrix of cells on the first conductive layer. In another embodiment each of the metal oxide cells is substantially square shaped, substantially circular shaped, substantially hexagonal shaped or substantially rectangular shaped. In another embodiment the separating means are banks. 
     In one embodiment the electrochemical cell further comprises: an electrocatalytic layer formed the electrolyte and the second conductive layer. In another embodiment the electrocatalytic layer is any one of platinum, ruthenium, rhodium, palladium, iridium or osmium. 
     In one embodiment the electrochemical cell further comprises: a first insulating substrate on a side of the first conductive layer opposite to the metal oxide layer. In another embodiment the electrochemical cell further comprises: a second insulating substrate on a side of the second conductive layer opposite to the electrolyte. In a further embodiment at least one of the first and second insulating substrates is glass or plastic. In a further embodiment the metal oxide layer is a semiconductor. 
     In one embodiment the metal oxide particle dispersion liquid comprises a plurality of titanium dioxide particles. In another embodiment the metal oxide particle dispersion liquid comprises a plurality of metal oxide particles, and wherein the functional dye layer is formed on a surface of the metal oxide particles. In another embodiment the first and second conductive layers are continuous layers. In another embodiment the first conductive layer is a transparent conductive oxide layer. In another embodiment the second conductive layer is a transparent conductive oxide layer. 
     In one embodiment the electrochemical cell is a dye sensitised solar cell. In another embodiment the electrochemical cell is an electrochromic display. In a further embodiment the functional dye layer is an electrochromophore layer. 
     In a second embodiment of the present invention a method of forming an electrochemical cell is provided. The method of forming an electrochemical cell comprising: forming a first conductive layer; forming a metal oxide layer from a metal oxide particle dispersion liquid on the first conductive layer, the metal oxide layer comprising a plurality of adjacent metal oxide cells, spaced from one another; forming a functional dye layer on the metal oxide layer; forming a second conductive layer; and providing an electrolyte between the functional dye layer and the second conductive layer, wherein at least one of the first and second conductive layers is transparent. 
     In one embodiment the method further comprises: forming separating means on the first conductive layer surrounding each of the plurality of adjacent metal oxide cells. In another embodiment the metal oxide layer is inkjet printed onto the first conductive layer. In another embodiment the metal oxide layer is inkjet printed onto the first conductive layer in one step. 
     In one embodiment the method further comprises: providing an electrocatalytic layer between the electrolyte and the second conductive layer. In another embodiment the method further comprises: forming the first conductive layer on a first insulating substrate, whereby the first insulating substrate and the metal oxide layer are on opposite sides of the first conductive layer. In another embodiment the method further comprises: forming the second conductive layer on a second insulating substrate, whereby the second insulating substrate and the electrolyte are on opposite sides of the second conductive layer. 
     In a further embodiment the metal oxide layer is formed from a water based metal oxide particle dispersion liquid. In another embodiment the metal oxide layer is formed from an alcohol based metal oxide particle dispersion liquid. 
     The method of fabrication of the electrochemical cell of the present invention, using inkjet printing, is advantageous over screen printing fabrication as format scaling (up or down) does not require re-investment in machine hardware. This is because inkjet fabrication is software controlled and the software can be reconfigured without the expense of commissioning new screens. Additionally, inkjet heads are significantly more durable, than patterned screens, as patterned screens last only approximately 100 uses. 
     Furthermore, the drop on demand placement enabled by inkjet fabrication is less wasteful than screen printing. Unlike conventional inkjet overwriting, where each deposited layer is dried and then printed over to produce a thick deposition, the inkjet flood filling technique, which doses a confined region with a large volume of liquid to provide the required deposit thickness, has been shown to produce fracture-free metal oxide layers. Moreover, the surface confinement used to enable flood filling, through the use of a bank structure, ensures long range uniform material distribution and therefore uniform and repeatable performance. Additionally, surface confinement through the use of a bank structure ensures enhanced picture quality and contrast by colour separation between the different coloured cells. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described by way of further example only and with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates a typical Dye Sensitised Solar Cell (DSSC) of the prior art; 
         FIG. 2  illustrates a further DSSC of the prior art; 
         FIG. 3  illustrates an electrochemical cell of the present invention; 
         FIG. 4  illustrates a process flow diagram for the fabrication of an electrochemical cell of the present invention; and 
         FIG. 5  illustrates several pixel cells of a bank structure filled with metal oxide. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to an electrochemical cell such as a Dye Sensitised Solar Cell (DSSC) or an electrochromic display (ECD). One electrochemical cell  400  of the present invention comprises, with reference to  FIG. 3 , a first transparent insulating substrate layer  401 ; a first transparent conductive oxide (TCO) electrode layer  402 ; a metal oxide layer  403 ; a sensitiser (dye)/electrochromic material layer  404 ; an electrolyte layer  405 ; a second TCO electrode layer  406 ; and a second transparent insulating substrate layer  407 . 
     The first and second transparent insulating substrate layers  401 ,  407  are preferably glass or plastic. The metal oxide layer  403  is preferably titanium dioxide (TiO 2 ) and is a semiconductor. 
     The metal oxide layer  403  should preferably be a material which promotes intimate adhesion of the sensitiser (dye)/electrochromic material layer  404  on its surface. Additionally, the particles of the metal oxide layer  403  must be reasonably light transmissible. Particles greater then 500 nm are expected to be opaque and are not generally considered appropriate for use in the present invention. Such large particles would also tend to cause inkjet nozzle blocking. 
     In a first embodiment of the present invention, a bank structure  410  is formed on the first TCO layer  402 , prior to the application of the metal oxide layer  403 , so that a metal oxide layer  403  is formed of isolated cells. In one embodiment the bank structure  410  may be formed from a polymer or a polyimide. 
     Preferably, the bank structure is hydro- and/or oleophobic in some part while the TCO layer  402  is hydro- and/or oleophilic, depending on the nature of the metal oxide ink used to form the metal oxide layer  403 . 
     The bank structure  410  can take on any desired shape forming a matrix of individual pixel cells on the first TCO layer  402 , within which the isolated metal oxide cells are formed; such that no metal oxide bridges the bank structure  410  to cause short circuiting. 
     When the electrochemical cell is an ECD, it is essential that all the metal oxide cells (pixels) are electrically isolated from one another to control the image formation. While the metal oxide cell electrical isolation is not essential when the electrochemical cell is a DSSC, it is preferable to maintain a uniform metal oxide distribution throughout the active device area. 
     The ECD electrochemical cell can be considered as being composed of a plurality of micro-electrochemical cells, and different micro-electrochemical cells may have different coloured electrochromophore layers  404 . Each micro-electrochemical cell is separated from the other micro-electrochemical cells, which together form the ECD, by the bank structure  410 . Each micro-electrochemical cell is preferably between 20 μm to 500 μm across. 
     In a further embodiment of the present invention an electrocatalytic layer can be formed between the electrolyte layer  405  and the second TCO layer  406 . The electrocatalytic layer is preferably greater than 2 nm thick and is selected to enhance the electrolyte regeneration. In the case of the DSSC, effective electrocatalytic metals can be selected from the platinum group metals; platinum, ruthenium, rhodium, palladium, iridium or osmium. The use of an electrocatalytic layer improves the overall performance of the electrochemical cell of the present invention. 
     The present invention also relates to a method of fabricating the electrochemical cell  400  of the present invention.  FIG. 4  illustrates a process flow diagram for the fabrication of an electrochemical cell  400  of the present invention. 
     The TCO layer  402  is formed on the first transparent insulating substrate layer  401 ,  FIG. 4   a . Preferably, the TCO layer  402  has a sheet resistivity of 8-10 Ω.sq. and is made of indium tin oxide or fluorine doped tin oxide. Fluorine doped tin oxide is preferable due to its cheapness and inertness during the high temperature sintering stage. 
     The bank structure  410  is then fabricated on the TCO layer  402 ,  FIG. 4   b . In the first embodiment of the present invention, the bank structure  410  forms a matrix of square pixel cells. In order to form the bank structure  410  on the TCO layer  402 , a photo-reactive polyimide source material is coated on to the TCO layer  402  and dried. A mask, in the shape of the matrix of pixel cells is then applied to the TCO layer  402 . An ultraviolet (UV) light is irradiated through the mask to cause cross-linking of the polyimide in the exposed regions. The unexposed regions are removed by chemical developing, and the bank structure  410  is thermally cured. 
     The TCO layer  402  having a bank structure  410  is then treated by oxygen or oxygen plus carbon tetrafluoride plasma to remove residual polyimide in the exposed regions. A carbon tetrafluoride (CF 4 ) plasma treatment is then applied to cause the polyimide bank structure  410  to become hydrophobic, while preserving the hydrophilic nature of the TCO layer  402 . 
     The metal oxide layer  403  is then inkjet printed onto the TCO layer  402  having the bank structure  410  formed thereon. The metal oxide ink is jetted into each of the isolated pixel cells to form the metal oxide layer  403 ,  FIG. 4   c , from an inkjet head. In one embodiment, a water-based aqueous colloidal metal oxide liquid ink of concentration ≦10% volume fraction (v/v) is used. In an alternative embodiment, an alcohol-based colloidal metal oxide liquid ink is used. In both embodiments, preferably, the metal oxide particles dispersed in the liquid are titanium dioxide (TiO 2 ) particles and have a diameter of approximately 18 nm. However, the diameter of the particles can be selected from a range of 5 nm to 500 nm, although the maximum diameter of particles is limited by the inkjet head characteristics. 
     Other additives can be added to the metal oxide ink in order to ensure compatibility of the metal oxide ink with the inkjet head and thus improve the stability and accuracy of the metal oxide ink ejection from the inkjet head. For example, a viscosity enhancer, such as a polyethylene glycol (PEG), may be added to the metal oxide ink. In addition, a surfactant, such as TRITON-X may be added to the metal oxide ink in order to adjust the surface tension of the ink 
     Other additives can also be added to the metal oxide ink to control the structure of the dried deposit after inkjet printing. For instance, polyethylene oxide (PEO) can be used as a matrix binder material, intercalating the metal oxide particles contained in the ink as the solvent evaporates from the target surface. Subsequent high temperature air anneals would remove this volatile intercalated material to leave a metal oxide film of the desired porosity. 
     The volume of all dry material included in these metal oxide inks should be no greater than one quarter of the volume of metal oxide particles in the same solution. Above this fraction the residual material will exceed the volume created between the metal oxide particles, assuming a close packing arrangement of identically sized particles, and will disrupt the electrical connectivity between them upon drying. By example, an aqueous 10% v/v TiO 2  solution should contain no more than 2% w/w PEO. Neglecting the contribution of the TiO 2 , this equates to a pure 3% w/w PEO solution. With such a high PEO concentration as this, any resultant ink to be too viscous to inkjet print normally. Hence, we do not expect the usefulness of inks to be limited by the volumetric fraction of dry material additives, but by the effect that such additives have on the resultant ink physical properties, predominantly viscosity. 
     It is possible that the function of metal oxide ink viscosity modification and porosity/binding control can be performed by a single additive. Both PEG and PEO are examples of such materials. 
     After inkjet printing the metal oxide deposit is dried and then sintered in air at 300 C to provide the metal oxide layer  403 . 
     The thickness of the metal oxide layer  403  is controlled by the concentration of the metal oxide ink, and the deposition volume. The resultant deviation in the peak thickness of the metal oxide layer  403  is less than 1.5% between pixel cells over a 50 cm 2  substrate area. 
     The substrate layer  401  comprising the TCO layer  402 , the bank structure  410  and the metal oxide layer  403  is then immersed in sensitiser (dye)  404  for a period of time. The sensitiser (dye)  404  is thereby absorbed onto the surface of the metal oxide layer  403 ,  FIG. 4   d . For the DSSC example, the substrate was immersed in a 0.3 mM solution of N719 (obtained from Solaronix) in dry ethanol for 24 hours, After immobilisation of the sensitiser (dye)  404 , the substrate is rinsed in ethanol and blown dry using nitrogen. 
     The first TCO layer  402 , having the porous metal oxide layer  403  and sensitiser (dye) layer  404  formed thereon, is then assembled with the second TCO layer  406 . Both electrode layers  402 ,  406  are sandwiched together with a perimeter spacer  411  to create an electrode-to-electrode gap, before filling with the electrolyte layer  405  ( FIG. 4   e ). Once the electrolyte layer  405  is introduced, the DSSC is completed by sealing the remaining aperture. 
     If an electrocatalytic layer is desired in the electrochemical cell of the present invention, then the electrocatalytic layer is formed on the second TCO layer  406  prior to the electrode layers  402 ,  406  being sandwiched together. 
     An inkjet head is capable of providing a well defined aqueous colloidal metal oxide ink droplet, with volume deviation less than ±1.5%, to a precise location on the TCO layer  402 . Moreover, this volumetric accuracy of ≦1.5% represents that for a commercial printer head. Several industrial heads and complementary techniques are available which can reduce this figure to ≦1%. 
     Inkjet deposition enables accurate positioning of the metal oxide on the TCO layer  402 , within each pixel cell of the bank structure  410  as required. Thus, the thickness of the metal oxide layer  403  can be controlled precisely and a uniform porous metal oxide layer  403  can be obtained. 
     When at least part of the bank structure  410  is hydro- and/or oleophobic, and at least part of the TCO layer  402  is hydro- and/or oleophilic, the bank structure  410  repels the deposited metal oxide ink, thus correcting the final position of the deposited metal oxide ink droplets on the target surface and compensating for the inherent ±15 μm droplet lateral divergence from the inkjet nozzle axis. This repulsion is especially beneficial in the case of the ECD to prevent pixel short-circuits caused by metal oxide  403  bridging the bank structure  410 . The bank structure  410  also enables the formation of a narrower gap between ECD pixels than otherwise permitted by the 30 μm spacing necessary for bank-less free-printing, enabling a higher active area ratio to be obtained in the ECD and increased image quality. 
     The metal oxide layer  403  should be several microns thick to function effectively. In traditional inkjet printing the thickness of the ink is built up to the desired thickness by using an overwriting technique, wherein each deposited layer is dried and sintered and then overwritten with another layer of ink, and so on, until the desired thickness is reached. 
     However, the method of the present invention uses a flood filling technique, whereby a large volume of metal oxide ink is introduced into each pixel cell of the bank structure  410  in one pass. The bank structure  410  prevents the metal oxide ink from spreading into neighbouring pixel cells. Using this process, only a single drying and sintering stage is required to produce the desired thickness of the metal oxide layer  403 . 
       FIG. 5  illustrates several pixel cells of a bank structure  410  filled with metal oxide. 
     A bank structure  410  having a matrix of square pixel cells produces a quasi-pyramidal dry metal oxide topography when the flood filling technique is used to fill each pixel cell with metal oxide ink. The bank structure  410  acts to confine the deposited metal oxide ink to a local region, within the pixel cells on the TCO layer  402 . Without this confinement, the metal oxide ink would be distributed freely across the TCO layer  402  following deposition and would form a continuous metal oxide layer  403 . 
     The bank structure  410  of the present invention increases the metal oxide layer&#39;s  403  ability to accommodate bending stress without fracturing, compared to a continuous metal oxide layer  403 . This enables a flexible substrate  401  to be utilised, such as a plastic first insulating substrate  401 . 
     In the first embodiment of the present invention, the bank structure  410  comprises a matrix of square pixel cells as illustrated in  FIG. 5 . However, the pixel cells are not limited to being square. When the electrochemical cell  400  of the present invention is an ECD, square pixels are preferred as they are compatible with active matrix backplane fabrication technology. However, when the electrochemical cell  400  of the present invention is a DSSC, several different pixel cell shapes can be used, such as a hexagonal, rectangular, circular or square pixel cell shape can be used. A hexagonal pixel cell shape or a square pixel cell shape is preferable for use in a DSSC of the present invention. 
     DSSC&#39;s of the present invention have been made with an energy conversion efficiency (η), an open circuit voltage (V oc ), a short circuit current (I sc ) and a fill factor (FF) of 5.0%, 0.48 V, 15 mA/cm 2  and 56%, respectively. The variation in energy conversion efficiency of a electrochemical cell of the present invention over a 50 cm 2  substrate area is less than 1.5%. This is due to the process stability of the inkjet fabrication method of the present invention. 
     Wider bank structures  410  are deleterious to both ECD operation, by a reduction in image quality, and DSSC operation, by a reduction in efficiency; resulting from a decrease in active area. Therefore, the bank structure  410  has a preferable width from 0.2 μm to 20 μm. 0.2 μm is the resolution limit for cost effective fabrication of the bank structure  410  by photolithography. 20 μm is considered the maximum effective bank structure  410  width before serious degradation of the image and performance becomes inhibitive, compared to the lowest common display resolutions of 72 dpi. Using inkjet technology hydrophilic pixel cell sizes less than 1 mm 2  are readily achievable, though lengths less than several hundred microns are preferred. 
     In the case of DSSC, absorption of light is proportional to the thickness of the porous metal oxide layer  403 . If too thin, a fraction of the incident light will pass unhindered through the metal oxide layer  403 , with a loss of potential efficiency. If too thick, once all of the useful light has been completely absorbed, any remaining metal oxide layer  403  thickness will be redundant. Therefore, preferably the thickness of the deposited metal oxide layer  403  should be between from 0.5 μm to 20 μm. 
     Moreover, due to the uniformity of the thickness of the metal oxide layer  403  produced by inkjet printing over screen printing, the optimal metal oxide layer  403  thickness can be thinner when using inkjet printing. 
     Furthermore, in the case of screen printing, the ink viscosity must be much higher than that preferred for inkjet printing. Therefore, the material added to increase viscosity must be removed during the sintering process. Consequently, the as-deposited, pre-sintered metal oxide layer  403  thickness must be greater for screen-printing than for inkjet printing. 
     Although a bank structure  410  is used to form a matrix of isolated pixel cells on the TCO layer  402 , prior to application of the metal oxide ink, the present invention is not limited to banks. Any method of forming isolated pixel cells on the TCO layer  402  may be used, such by creating troughs in the TCO layer  402 . 
     Additionally, although the sensitiser (dye)  404  is formed on the metal oxide layer  403  by immersion of the metal oxide layer  403  in the sensitiser (dye)  404  for a predetermined period of time, the sensitiser (dye)  404  may be formed on the metal oxide layer using different techniques. For example, the sensitiser (dye)  404  may be ink jet printed onto the metal oxide layer  403  following formation of the metal oxide layer  403 . 
     Furthermore, it is not essential for the first transparent conductive oxide layer  402  to be formed of an oxide material for the electrochemical cell of the present invention to function. Additionally, it is not essential for the second transparent conductive oxide layer  406  to be transparent or formed of an oxide material for the electrochemical cell of the present invention to function. Indeed, it is not essential to provide the second substrate (or either substrate in the finished device). 
     Any suitable material or process can be used for forming the bank structures. However, it is preferred to deposit them as a polymer, and more preferably as a polyimide, pattern. 
     Although liquid electrolytes have been discussed above, solid or gel electrolytes are also suitable for use in the present invention and, in this context, any reference in this specification to providing an electrolyte between an electrode/conductive layer and another element includes forming the electrode/conductive layer and/or the other element on the electrolyte. 
     The foregoing description has been given by way of example only and it will be appreciated by a person skilled in the art that modifications can be made without departing from the scope of the present invention.