Abstract:
A method of forming an electrochemical cell includes the steps of disposing a separating material on a first conductive material, disposing a metal oxide on the first conductive material of an opening of the separating material, and disposing a dye on the metal oxide.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an electrochemical cell structure and a method of fabrication. Preferably, the present invention relates to an ink used in the generation of a coloured electrochemical cell structure. 
       BACKGROUND OF THE INVENTION 
       [0002]    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 lame surface exposure requirements. 
         [0003]    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. 
         [0004]    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 . 
         [0005]    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. 
         [0006]    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%. 
         [0007]    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. 
         [0008]    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). 
         [0009]    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, lame 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. 
         [0010]    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. 
         [0011]    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 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 % veil 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 . 
         [0012]    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 . 
         [0013]    The first transparent conductive oxide (TCO) electrode layer  1  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. 
         [0014]    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. 
         [0015]    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. 
         [0016]    U.S. Pat. No. 5,830,597, entitled “Method and Equipment for Producing a Photochemical Cel”, 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 . 
         [0017]    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. 
         [0018]    U.S. Pat. No. 6,310,282B1, entitled “Photovoltaic Conversion Element and a Dye-Sensitising Photovoltaic Cel”, by Sakurai et al. discloses a multi-colour DSSC  200 . As illustrated in  FIG. 3 , the multi-colour DSSC  200  comprises a first transparent electrode layer  201 ; a transparent semiconductor layer  202  of TiO 2 , sensitising dye absorption portions  203 ,  204 ,  205 ,  206 , of four colours, absorbed on the surface of the transparent semiconductor layer  202 ; a carrier transport layer  207 ; a second transparent electrode layer  208 ; and auxiliary electrodes  209  attached to the first and second electrode layers  201 ,  208 . 
         [0019]    The multi-colour DSSC  200  is manufactured by covering the transparent semiconductor layer  202  with a mask having openings that coincide only with the red sensitising dye absorption portions  203 , dipping the transparent semiconductor layer  202  in red sensitising dye for a predetermined period of time, removing the transparent semiconductor layer  202  from the red sensitising dye and removing the mask. Then covering the transparent semiconductor layer  202  with a mask having openings that coincide only with the green sensitising dye absorption portions  204 , dipping the transparent semiconductor layer  202  in green sensitising dye for a predetermined period of time, removing the transparent semiconductor layer  202  from the green sensitising dye and removing the mask. Next covering the transparent semiconductor layer  202  with a mask having openings that coincide only with the blue sensitising dye absorption portions  205 , dipping the transparent semiconductor layer  202  in blue sensitising dye for a predetermined period of time, removing the transparent semiconductor layer  202  from the blue sensitising dye and removing the mask. Finally, dipping the transparent semiconductor layer  202  in black sensitising dye for a predetermined period of time and removing the transparent semiconductor layer  202  from the black sensitising dye. 
         [0020]    The multi-colour DSSC  200  would have poor picture quality with respect to dye bleeding through the transparent semiconductor layer  202  from the separate dye absorption portions  203 ,  204 ,  205 ,  206  and the lack of greyscale (contrast) control. 
         [0021]    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. 
         [0022]    In the case of the ECD, screen-printing demands are further exacerbated by the requirement to create ever liner 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. 
         [0023]    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 
       [0024]    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 costs. Additionally, the present invention aims to produce a multi-coloured electrochemical cell having improved picture quality with respect to blurring (bleeding) and greyscale (contrast) over that of the prior art. 
         [0025]    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 provided 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 provided on the metal oxide layer; a second conductive layer; and an electrolyte provided 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 functional dye layer is formed from an organic solvent ink. 
         [0026]    In one embodiment the organic solvent ink comprises a first solvent and a second solvent. In another embodiment the first solvent and the second solvent have different boiling points. In another embodiment the first solvent has a boiling point greater than a boiling point of the second solvent. In another embodiment the first solvent has a boiling point greater than substantially 150° C. In another embodiment the first solvent is 5% v/v of the organic solvent ink and the second solvent is 95% v/v of the organic solvent ink. In another embodiment the first solvent is less than 40% v/v of the organic solvent ink. In another embodiment the first solvent is 1,3-dimethyl-2-imidazolidinone and the second solvent is 3-methyl-2-oxazolidinone. 
         [0027]    In a further embodiment the functional dye layer comprises at least two different coloured organic solvent inks, such that at least some of the plurality of adjacent metal oxide cell have different colours. In another embodiment at least one of the at least two different coloured organic solvent inks contributes to the electrochemical cell power generation. In another embodiment a depth of colour of the functional dye layer in at least one metal oxide cell is altered by altering a thickness of the metal oxide layer. In another embodiment a depth of colour of the functional dye layer in at least one metal oxide cell is altered by altering a size of droplets of the organic solvent ink formed thereon. In another embodiment a depth of colour of the functional dye layer in at least one metal oxide cell is altered by altering a number of droplets of the organic solvent ink formed thereon. In another embodiment a depth of colour of the functional dye layer in at least one metal oxide cell is altered by altering a concentration of the organic solvent ink formed thereon. 
         [0028]    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 a further embodiment the separating means are banks. 
         [0029]    In one embodiment the electrochemical cell further comprises: a reflector layer provided on the opposite side of the functional dye layer to the first conductive layer. In another embodiment the electrochemical cell further comprises: an electrocatalytic layer formed between the electrolyte and the second conductive layer. In another embodiment the electrocatalytic layer is any one of platinum, ruthenium, rhodium, palladium, iridium or osmium. 
         [0030]    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 another embodiment the first insulating substrate is glass or plastic. In another embodiment the metal oxide layer is a titanium dioxide layer. In another embodiment the metal oxide layer comprises particles of metal oxide, and wherein the functional dye layer is formed on a surface of the particles of the metal oxide layer. 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 another embodiment the electrochemical cell is a dye sensitised solar cell. In another embodiment the electrochemical cell is an electrochromic display. In another embodiment the functional dye layer is an electrochromophore layer. 
         [0031]    In a second embodiment of the present invention an electrochemical cell is provided. The electrochemical cell comprising: a first conductive layer; a metal oxide layer provided on the first conductive layer; a functional dye layer provided on the metal oxide layer; a second conductive layer; and an electrolyte provided 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 functional dye layer is formed from a binary solvent ink, comprising a first solvent and a second solvent. 
         [0032]    In one embodiment the first solvent and the second solvent have different boiling points. In another embodiment the first solvent has a boiling point greater than a boiling point of the second solvent. In another embodiment the first solvent has a boiling point greater than substantially 150° C. In another embodiment the first solvent is 5% v/v of the binary solvent ink and the second solvent is 95% v/v of the binary solvent ink. In another embodiment the first solvent is less than 40% v/v of the binary solvent ink. In another embodiment the first solvent is 1,3-dimethyl-2-imidazolidinone and the second solvent is 3-methyl-2-oxazolidinone. 
         [0033]    In a further embodiment a depth of colour of the functional dye layer in at least one metal oxide cell is altered by altering a thickness of the metal oxide layer. In another embodiment a depth of colour of the functional dye layer in at least one metal oxide cell is altered by altering a size of droplets of the organic solvent ink formed thereon. In another embodiment a depth of colour of the functional dye layer in at least one metal oxide cell is altered by altering a number of droplets of the organic solvent ink formed thereon. In another embodiment a depth of colour of the functional dye layer in at least one metal oxide cell is altered by altering a concentration of the organic solvent ink formed thereon. In another embodiment the first conductive layer is transparent, and the electrochemical cell further comprises: a reflector layer provided on the opposite side of the functional dye layer to the first conductive layer. 
         [0034]    In one embodiment the electrochemical cell further comprises: an electrocatalytic layer formed between 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 another 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 another embodiment the first insulating substrate is class or plastic. In another embodiment the metal oxide layer is a titanium dioxide layer. 
         [0035]    In a further embodiment the metal oxide layer comprises particles of metal oxide, and wherein the functional dye layer is formed on a surface of the particles of the metal oxide layer. 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 another embodiment the electrochemical cell is a dye sensitised solar cell. In another embodiment the electrochemical cell is an electrochromic display. 
         [0036]    In a third embodiment of the present invention a method of forming an electrochemical cell is provided. The method comprising: forming a first conductive layer; forming a metal oxide layer 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 formed from an organic solvent ink 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. 
         [0037]    In one embodiment the organic solvent ink comprises a first solvent and a second solvent. In another embodiment the first solvent and the second solvent have different boiling points. In another embodiment the first solvent has a boiling point greater than a boiling point of the second solvent. In another embodiment the first solvent has a boiling point greater than substantially 150° C. In another embodiment the first solvent is 5% v/v of the organic solvent ink and the second solvent is 95% v/v of the organic solvent ink. In another embodiment the first solvent is less than 40% v/v of the organic solvent ink. In another embodiment the first solvent is 1,3-dimethyl-2-imidazolidinone and the second solvent is 3-methyl-2-oxazolidinone. 
         [0038]    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 another embodiment the organic solvent ink is inkjet printed onto the adjacent metal oxide cells of the metal oxide layer. In another embodiment the first conductive layer is transparent, and the method further comprises: providing a reflector layer on the opposite side of the functional dye layer to said at least one transparent conductive layer. 
         [0039]    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 first conductive layer. 
         [0040]    In a fourth embodiment of the present invention a method of forming an electrochemical cell is provided. The method comprising: forming a first conductive layer; forming a metal oxide layer on the first conductive layer; forming a functional dye layer formed from a binary solvent ink comprising a first solvent and a second solvent 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. 
         [0041]    In one embodiment the first solvent and the second solvent have different boiling points. In another embodiment the first solvent has a boiling point greater than a boiling point of the second solvent. In another embodiment the first solvent has a boiling point greater than substantially 150° C. In another embodiment the first solvent is 5% v/v of the binary solvent ink and the second solvent is 95% v/v of the binary solvent ink. In another embodiment the first solvent is less than 40% v/v of the binary solvent ink. In another embodiment the first solvent is 1,3-dimethyl-2-imidazolidinone and the second solvent is 3-methyl-2-oxazolidinone. 
         [0042]    In a further 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 another embodiment the binary solvent ink is inkjet printed onto the adjacent metal oxide cells of the metal oxide layer. In another embodiment the first conductive layer is transparent, and the method further comprises: providing a reflector layer on the opposite side of the functional dye layer to the at least one transparent conductive layer. 
         [0043]    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 one 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 first conductive layer. 
         [0044]    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. 
         [0045]    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 tilling 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 tilling, 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 by colour separation between the different coloured cells. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0046]    Embodiments of the present invention will now be described by way of further example only and with reference to the accompanying drawings, in which: 
           [0047]      FIG. 1  illustrates a typical Dye Sensitised Solar Cell (DSSC) of the prior art; 
           [0048]      FIG. 2  illustrates a further DSSC of the prior art; 
           [0049]      FIG. 3  illustrates a multi-colour DSSC of the prior art; 
           [0050]      FIG. 4  illustrates an electrochemical cell of a first embodiment of the present invention; 
           [0051]      FIG. 5  illustrates a process flow diagram for the fabrication of an electrochemical cell of the present invention; 
           [0052]      FIG. 6  illustrates an electrochemical cell of a second embodiment of the present invention; and 
           [0053]      FIG. 7  illustrates a square pixel cell having a pyramidal metal oxide topography. 
       
    
    
     DETAILED DESCRIPTION 
       [0054]    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. 4 , 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 . 
         [0055]    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. 
         [0056]    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. 
         [0057]    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. 
         [0058]    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 . 
         [0059]    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. 
         [0060]    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. 
         [0061]    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. 
         [0062]    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. 
         [0063]    The present invention also relates to a method of fabricating the electrochemical cell  400  of the present invention.  FIG. 5  illustrates a process flow diagram for the fabrication of the electrochemical cell  400  of the present invention. 
         [0064]    The TCO layer  402  is formed on the first transparent insulating substrate layer  401 ,  FIG. 5   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. 
         [0065]    The bank structure  410  is then fabricated on the TCO layer  402 ,  FIG. 5   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 at 350° C. 
         [0066]    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 . 
         [0067]    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. 5   c . Preferably, aqueous colloidal titanium dioxide (TiO 2 ) inks of ≦10% volume fraction (v/v) are used, containing particles &lt;500 nm in diameter. Other additives can be included in the metal oxide ink to ensure compatibility of the solution with the inkjet head. After deposition, the metal oxide layer  403  is dried and then sintered in air at ≧300° C. 
         [0068]    Precise control of the metal oxide layer  403  thickness throughout the device area is essential to provide a uniform canvas on which the DSSC or ECD image can be formed. The thickness of the metal oxide layer  403  is controlled by the concentration of the aqueous colloidal TiO 2  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. 
         [0069]    Sensitiser (dye)  404  is then inkjet printed into each of the pixel cells, onto the metal oxide layer  403  formed therein. This method of fabrication allows for the formation of different coloured pixel cells, by the application of different coloured sensitiser (dye)  404  to different pixel cells, in order to create a coloured image, with each pixel cell of the bank structure  410  corresponding to a pixel of a picture. Therefore, a high image quality electrochemical cell, with a resolution of 200 dots per inch (dpi) or greater, can be created. The different coloured sensitiser (dye)  404  is absorbed by the metal oxide layer  403 .  FIG. 5   d . After immobilisation of the sensitiser (dye)  404 , the excess sensitiser (dye)  404  and remaining solvent is removed by rinsing the complete structure in ethanol and blowing dry in nitrogen. 
         [0070]    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 to create an electrode-to-electrode gap, before filling with the electrolyte layer  405 . Once the electrolyte layer  405  is introduced, the DSSC is completed by sealing the remaining aperture. 
         [0071]    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. 
         [0072]    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%. 
         [0073]    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. 
         [0074]    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. 
         [0075]    The metal oxide layer  403  should be several microns thick to function effectively. In traditional inkjet printing the thickness of the deposit is built up to the desired profile 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. 
         [0076]    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 . The bank structure  410  also prevents the different coloured sensitiser (dye)  404  from spreading into neighbouring pixel cells, thus preventing colour bleeding. 
         [0077]    The sensitiser (dye) ink solution is absorbed into the porous metal oxide layer  403  through capillary action. If the bank structure  410  was not used to separate the metal oxide layer  403  into isolated pixel cells, then upon jetting from an ink jet head the sensitiser (dye)  404  would spread throughout the metal oxide layer  403  and picture quality would be reduced. 
         [0078]    Using a continuous metal oxide layer  403  makes control of image colour depth difficult by inkjet dye overwriting, as the deposited ink merely spreads throughout the porous metal oxide layer  403 . To increase the resolution and contrast between the colours and to enable control of sensitiser (dye)  404  colour depth, the metal oxide layer  403  is separated into isolated pixel cells, so that sensitiser (dye)  404  introduced into one cell remains within that cell. Thus, the bank structure  410  is used to prevent lateral liquid diffusion. 
         [0079]    Additionally, the use of the bank structure  410  enables the metal oxide layer  403  to have a uniform and repeatable cross-sectional profile throughout, assisting uniformity of colour depth across the image. To provide the required image quality, the pixel cells defined by the bank structure  410  should be as small as practicable. 
         [0080]    In the case of the DSSC, not all of the coloured pixel cells are required to be coloured using sensitiser (dye)  404 , however, at least some of the colours used in the image should preferably be coloured using sensitiser (dye)  404 . The remainder pixel cells of the image can be fabricated using ‘inert’ dye that contributes to the image but not the electrochemical cell power generation. Though described as ‘inert’ dyes, they should chemically bond to the metal oxide layer  403  to provide good image stability. If only one coloured sensitiser (dye)  404  is used, black is preferable. This is because black sensitiser (dye)  404  has a high energy conversion efficiency. 
         [0081]    However, it is preferable that all the pixel cells are coloured using sensitiser (dye)  404  that contributes to the electrochemical cell power generation to maximise power output. This is because the use of a plurality of isolated pixel cells reduces the energy conversion efficiency of the electrochemical cell of the present invention by area. Additionally, if only some of the pixel cells are coloured using sensitiser (dye)  404  that contributes to the electrochemical cell power generation, then the energy conversion efficiency of the electrochemical cell of the present invention is further reduced. 
         [0082]    The depth of colour of the sensitiser (dye)  404  can be controlled through adjustment of the size and number of droplets printed into each pixel cell of the bank structure  410 . Additionally, the depth of colour of the sensitiser (dye)  404  can be controlled through adjustment of the thickness of the metal oxide layer  403 . 
         [0083]    At least four inks are necessary to provide photographic quality images; cyan, magenta, yellow and black or red, green, blue and black depending on the image construction. Many of these coloured sensitisers (dyes)  404  already exist, i.e. R II (dmbpy) 2 (dcbpy)Cl 2  is a known yellow dye, N719 is a known purple/red dye and Fe II  complexes are known blue dyes. To create a colour mix within one pixel cell, a predefined dose of one colour can be jetted into that pixel cell before being overwritten with a different dose of another colour. 
         [0084]    Known DSSC immersive solutions comprise a dye dissolved in a low boiling point solvent in order to immobilise the dye molecules on a metal oxide surface. However, these solutions are not suitable for use with inkjet printing technology because the rapid evaporation of the solvent, that would occur at the inkjet head nozzle plate, can cause solute to build up around the nozzles causing them to eventually clog. 
         [0085]    Furthermore, the volume of liquid that can be deposited on the device surface in a single pass by inkjet printing is very small, typically 0.5 nl/pixel, and would rapidly evaporate to dryness at room temperature if a low boiling point solvent was used; taking less than 10 seconds to completely dry. This timescale is too short compared to that required for the dye to be chemically absorbed on to the metal oxide surface, typically 24 hours. Hence, typical immersive dye solutions are unsuitable for inkjet deposition. 
         [0086]    Alternatively, inkjet printable inks could be formed by dissolving the dyes in high boiling point solvents, alleviating the nozzle clogging and printed surface drying problems. However, high boiling point solvents are often harmful and typically viscous making them difficult to inkjet print. Their high viscosity could also reduce the penetration of the deposited ink into the metal oxide film. Furthermore, it would be difficult to remove the solvent from the device surface after the dye is fixed on the metal oxide surface without heating to temperatures that could damage the dye molecules. 
         [0087]    Assuming that a “perfect solvent”, with low viscosity, being easily removable, with a high boiling point, could be identified, then after printing, it would have to remain resident on the metal oxide surface long enough for the dye to chemisorb, typically 24 hours for a dye concentration of 0.3 mM. While this timescale is achievable, it is not advantageous for mass production, where each process step should be complete within minutes, rather than hours. One way of overcoming this problem is to increase the dye concentration. Literature reports have shown that the dye absorption time reduces from 24 hours to 10 minutes by increasing the dye concentration from 0.3 mM to 21 mM. 
         [0088]    While such a high concentration ink would improve (reduce) the dye fixing time on the deposition surface, this conceptual ink would reduce the printed image greyscale graduation, in that 33 droplets of 0.3 mM ink would contain the same dye loading as 1 droplet of 10 mM concentration. 
         [0089]    Hence, even if the “perfect solvent” could be found, the image quality would be significantly degraded due to the lack of greyscale control caused by having to use a high dye concentration to reduce the dye immobilization time on the metal oxide layer. 
         [0090]    Therefore, in a preferred embodiment of the present invention, the sensitiser (dye)  404  is dissolved in a binary solvent solution to form inkjet printable inks; comprising a low boiling point bulk solvent and a high boiling point residual solvent. For example, a binary solvent ink may comprise 1 mM of N719 dye within 5% v/v of DMI (1,3-dimethyl-2-imidazolidinone) solvent and 95% v/v of NMO (3-methyl-2-oxazolidinone) solvent. NMO has a boiling point of 88° C. and DMI has a boiling point of 226° C. 
         [0091]    Preferably, the residual solvent has a boiling point substantially greater than 150° C. Furthermore, preferably, the residual solvent is less than 40% v/v of the binary solvent ink. 
         [0092]    When a binary solvent ink of the present invention is used in inkjet printing, following ejection of the binary solvent ink from a nozzle, the bulk solvent evaporates very quickly leaving behind the residual solvent on the surface. Since the residual solvent has a high boiling point, it remains liquid on the printed surface, ensuring that the dye has a prolonged period within which to bond with the metal oxide layer. Additionally, evaporation of the bulk solvent increases the dye concentration from approximately 1 mM to more than 10 mM, thereby reducing the dye absorption time, while preserving the inkjet greyscale control. In essence, the high concentration dye ink solution is formed on the target surface after printing. For instance, the above example of 1 mM N719 in 5% v/v DMI and 95% v/v NMO would be converted to 20 mM N719 in DMI after NMO evaporation from the printed surface. 
         [0093]    The binary solvent inks of the present invention assist inkjet printing, in as much that they do not clog the nozzles of an inkjet head. This is because although the bulk solvent can be readily evaporated from the nozzle region, the residual solvent remains in liquid form and prevents the formation of dry solute around the nozzles. 
         [0094]    Furthermore, the physical properties or the ink would be dominated by the bulk solvent, which possesses the lower boiling point and is more often less viscous than the higher boiling point residual solvent, making the composite ink much easier to tailor for inkjet printing. 
         [0095]    Finally, the binary ink solution permits accurate greyscale graduation control unlike the case described for a single high boiling point solvent ink, since we are effectively able to print fractional droplet volumes. For example, printing one droplet of the 1 mM binary solvent ink described above is equivalent to printing 0.05 droplets of 20 mM dye in pure DMI. 
         [0096]    The bulk solvent provides the necessary characteristics of the ink, whilst the residual solvent can be used to alter the concentration of the bulk solvent and enables the dye molecules to be immobilised on the surface of the metal oxide layer within a short process time. The process time of binary solvent inks, can be less than one tenth compared to the single solvent type inks. Furthermore, the use of a binary solvent ink provides stable ink ejection from an inkjet head. 
         [0097]    The dye which is mixed with the solvent(s) in order to create the colour of the ink must be soluble. Additionally, the solvents must be miscible and compatible with the dye. Furthermore, a surfactant can be added to the inks in order to adjust the surface tension and viscosity of the ink. 
         [0098]    In a further embodiment of the present invention an inert, diffuse, white reflector layer  412  can be provided between the sensitiser (dye) layer  404  and the second TCO layer  406 , as illustrated in  FIG. 6 , to enhance the electrochemical cell image quality. The white reflector layer  412  provides a clear background against which the image can be viewed. The white reflector layer  412  also causes the light path through the sensitiser (dye) layer  404  to double, increasing the efficiency of the electrochemical cell. The white reflector layer  412  may be formed of large (micron order) TiO 2  particles. It will be clear that the white reflector layer can be provided in other locations to provide this effect. For example, if the second conductor layer and the corresponding substrate are transparent, the reflector layer may be provided behind the substrate or between the substrate and the conductor layer. In addition, although preferred, it is not necessary for the reflector layer to be white—other colours can be used instead, depending on the desired picture. 
         [0099]    A bank structure  410  having a matrix of square pixel cells produces a quasi-pyramidal dry metal oxide topography as illustrated in  FIG. 7 , 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 of 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 . 
         [0100]    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 . 
         [0101]    In the first embodiment of the present invention, the bank structure  410  comprises a matrix of square pixel cells as illustrated in  FIG. 9 . 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 square, circular and hexagonal pixel cells. 
         [0102]    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 till 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. 
         [0103]    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. 
         [0104]    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. 
         [0105]    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. 
         [0106]    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. 
         [0107]    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 . 
         [0108]    Additionally, 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. Moreover, 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). 
         [0109]    Any suitable material can be used for the bank structures. However, it is preferred to deposit them as a polymer, and more preferably as a polyimide, pattern. 
         [0110]    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.