Abstract:
One limitation to the realisation of mass produced electrochemical cells is a lack of high resolution patterning techniques providing accurate alignment. A method of fabricating a patterned structure on a polymer layer for the manufacture of an electrochemical cell is provided. The method comprises: depositing a polymer layer upon a substrate; and stamping the polymer layer to form an embossed polymer layer using an embossing tool, the embossing tool having a first array of adjacent cells, spaced from one another and extending from the stamping face of the embossing tool and thereby forming a second array of adjacent cells, spaced from one another and extending as cavities within the embossed polymer layer.

Description:
FIELD OF THE INVENTION 
     The present invention relates, in general, to an electrochemical cell and its method of manufacture including micro-embossing and ink-jet printing steps. In particular, the present invention relates to the fabrication of a bank structure during manufacture of the electrochemical cell. 
     BACKGROUND OF THE INVENTION 
     A Dye-Sensitized Solar Cell (DSSC) functions as an electrochemical cell. 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 : a typical DSSC  10  comprises a substrate  1 ; a first electrode  2 ; a metal oxide layer  3 ; a functional dye layer  4 ; an electrolyte layer  5 ; a second electrode  6 ; and a second insulating layer  7 . 
     The DSSC  10  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, the sensitiser (dye)  4  is absorbed onto the surface of the metal oxide layer  3  to extend the light absorption range of the metal oxide layer  3  into the visible light region. 
     In order to increase the amount of light that the metal oxide layer  3  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 functional dye  4  resulting in increased light absorption and improving the energy conversion efficiency of the DSSC by more than 10%. 
     DSSC devices known in the art can be improved by fabricating the metal oxide layer as an array of micro-scale, high-density cells surrounded by barriers such as banks. In order to fabricate the banks, device fabrication techniques such as optical lithography, micro-embossing, optical interference lithography etc. can be employed because these techniques have become a key technology for mass production patterning techniques. Whilst these techniques allow for high-resolution patterning upon a substrate, tool alignment with previously defined structures upon the substrate is difficult. Accurate alignment is especially difficult in the case of large area, flexible substrates, due to the occurrence of warping, thermal expansion or shrinking of the substrate. Furthermore, in the case of roll-to-roll fabrication techniques, non-uniform distortions due to the necessary tensions applied to the substrate during transfer can cause further alignment difficulties. 
     One limitation to the realisation of mass produced DSSCs is therefore a lack of high resolution patterning techniques providing good alignment. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a method of fabricating a patterned structure on a polymer layer for the manufacture of an electrochemical cell, the method comprising: depositing a polymer layer upon a substrate; stamping the polymer layer to form an embossed polymer layer using an embossing tool, the embossing tool having a first array of adjacent cells, spaced from one and another and extending from the stamping face of the embossing tool and thereby forming a second array of adjacent cells, spaced from one and another and extending as cavities within the embossed polymer layer. 
     According to a second aspect of the present invention, there is provided a method of fabricating a patterned structure on a polymer layer for the manufacture of an electrochemical cell, the method comprising: depositing a conductive layer as a first electrode layer upon a substrate; depositing a polymer layer upon the first electrode layer; stamping the polymer layer to form an embossed polymer layer using an embossing tool, the embossing tool having a first array of adjacent cells, spaced from one and another and extending from the stamping face of the embossing tool, thereby forming a second array of adjacent cells, spaced from one and another and extending as cavities within the embossed polymer layer; and removing the remaining portions of the embossed polymer layer to reveal the first electrode layer within a plurality of cavities. 
     According to a third aspect of the present invention, there is provided method of fabricating a patterned structure on a polymer layer for the manufacture of an electrochemical cell, the method comprising: depositing a polymer layer and a conductive layer as first electrode upon a substrate; and stamping the polymer and conductive layers to form an embossed structure using an embossing tool, the embossing tool having a first array of adjacent cells, spaced from one another and extending from the stamping face of the embossing tool and thereby forming a second array of adjacent cells, spaced from one another and extending as cavities within the embossed polymer layer. 
     The present invention therefore provides new approaches of micro-embossing obviating or at least mitigating the problems associated with the prior art. The pre-patterned substrate effectively defines a necessary resolution, while the device components are built up by subsequent inkjet printing. 
     Preferred embodiments of the present invention are included in the dependent claims. 
    
    
     
       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  is a schematic diagram of a Dye-Sensitized Solar Cell (DSSC) as is known in the art; 
         FIG. 2  is a schematic diagram of a portion of an electrochemical cell according to a first embodiment of the present invention; 
         FIG. 3  is a schematic diagram of the fabrication of a tapered mold for forming an embossing stamp according to a second embodiment of the present invention; 
         FIG. 4  is a schematic diagram of the fabrication of a tapered bank structure using the embossing tool of  FIG. 3  according to a third embodiment of present invention; 
         FIG. 5  is a schematic diagram of the fabrication of a bank structure according to a fourth embodiment of the present invention. 
         FIG. 6  is a schematic diagram of the fabrication of a bank structure according to a fifth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Throughout the following description, like reference numerals identify like parts. 
     Referring to  FIG. 2 , a portion of an electrochemical cell according to a first embodiment of the present invention comprises a substrate wafer  20  having a conductive layer  22  as first electrode deposited thereon. A bank structure  24  is formed on the electrode layer  22  prior to the application of a metal oxide layer  26 . A discontinuous metal oxide layer is subsequently formed by inkjet printing the metal oxide into cells  28  to form an array of micro-scale, high density cells  28  surrounded by the banks  24  such that no metal oxide bridges the bank structure  24  to cause image deterioration by colour dye contamination. 
     Preferred embodiments of the present invention for the formation of bank structures or the like will now be described. 
     Referring to  FIG. 3 , the fabrication of a tapered mold for forming an embossing stamp  100  according to a second embodiment of the present invention comprises the following steps. In Step One, a silicon (Si) wafer  102  is coated with a silicon dioxide (SiO 2 ) layer  104  of around 200 nm to 300 nm thickness. In Step Two, optical lithography and plasma etching are applied to remove portions of the SiO 2  layer  104  and generate an array of windows across the SiO 2  layer  104 . A SF 6  or CF 4 +O 2  plasma is used to etch through the SiO 2  layer  104  and the wet etching is carried with potassium hydroxide aqueous solution (250 g/L) under 80° C. In Step Three, anisotropic wet etching is applied to obtain an array of tapered cavities  106  within the silicon wafer  102  at the locations of the SiO 2  windows. The sidewalls  108  of each cavity  106  are inwardly tapering and extend within the silicon wafer  102  to a flat cavity base  110 . On removal of the remaining SiO 2  layer  104  using a HF water solution, a silicon wafer template is obtained, which becomes a tapered mold  112 . In other words, the silicon wafer template has a plurality of recesses each of which has a tapered shape. 
     Stamp replication is illustrated in Step Four. A material, such as a metal, ceramic or polymer with high stiffness  114  is deposited by electroplating, casting, thermal evaporation or sputtering etc. onto the tapered mold  112 . In Step Five, the tapered mold  112  is removed, leaving the embossing stamp  100 . Step Five illustrates a side view and top view of the embossing stamp. 
     Referring to  FIG. 4  a schematic diagram of the fabrication of a tapered bank structure using the embossing stamp  100  of  FIG. 3  comprises the following steps. In Step One, a polymer layer  150 , around 2 μm thick and formed of a polymethyl methacrylate (PMMA) is spin-coated and baked at 120° C. and thereby deposited upon a substrate  152  such as a glass or polyethylene naphthalate (PEN). The embossing stamp  100  is subsequently brought against the polymer layer  150  at 160° C. under 20 bar pressure. The arrangement is subsequently cooled to room temperature and the embossing stamp  100  removed to provide an array of tapered cavities  106  within the polymer layer  150  separated by an array of banks  153 . 
     In Step Two, an electrode  154  is formed by depositing conductive material upon the surface of the polymer layer  150 . Also in Step Two, an O 2  plasma treatment is applied to provide a hydrophilic/lyophilic surface  156  upon the conductive layer  154 . 
     Subsequently, in Step Three, a Self-Assembled Monolayer molecule (SAM)  158 , such as 1H, 1H, 2H, 2H-perfluorodecyl-trichlorosilane solution (approximately 0.01 mol in hexane) for conductive oxide, is applied by soft contact printing. The SAM provides a hydrophobic surface  160  upon the uppermost portion of the banks  153  forming the array of tapered cavities  106 , while the flat cavity base  110  remains hydrophilic. In Step Four, a metal oxide semiconductor colloidal suspension  162  and functional dye solution  164  (such as ruthenium dye complex) is deposited into the cavities  106  using ink-jet printing techniques. 
     After drying, the device is completed by arranging a counter electrode (not shown) such as a Pt/ITO (indium tin oxide) coated glass or PEN at a 20 μm distance having an electrolyte inserted into the device. As an example, the electrolyte is a redox electrolyte such as an iodine and potassium iodine mixture in acetonitrile. 
     Referring to  FIG. 5 , a schematic diagram of the fabrication of a bank structure according to a fourth embodiment of the present invention comprises the following steps. In Step One, a glass or polyethylene naphthalate (PEN) is provided as a substrate  200  upon which is spin-coated a conductive layer  202 . A polymer layer  204 , around 2 μm thick and formed of polymethyl methacrylate (PMMA), is spin-coated and baked at 120° C. In Step Two, an embossing stamp  100  is subsequently brought against the polymer layer  204  at 160° C. under 20 bar pressure. The arrangement is subsequently cooled to room temperature and the embossing stamp  100  removed to provide an array of cavities  106  within the polymer layer  150  separated by an array of banks  153 . 
     For spin-coating deposition of conductive layer  202 , the prefabricated bank structure is not necessarily a tapered shape. In this case, the embossed structure can be fabricated by using an embossing stamp  100  with vertical sidewall although an embossing stamp having a tapered profile could equally be used. After embossing the polymer layer  204 , the remaining polymer layer  204  covering a base  110  of each cavity  106  is removed by plasma etching (Step Three). Steps Four and Five are the same as carried out and described above in Steps Three and Four in accordance with  FIG. 4 . 
     Referring to  FIG. 6 , a schematic diagram of the fabrication of another type of bank structure using micro-embossing in accordance with the present invention comprises the following steps. In Step One, a polymer layer (such as photoresist AZ 5214E)  310 , around 2 μm thick is spin-coated on the substrate  300  and baked at 120° C. Afterwards an approximately 100 nm thick Au layer  320  is thermally evaporated onto the resist surface. In Step Two, an embossing tool  330  is brought against the sample surface at 140° C. under 20 bar pressure. The arrangement is subsequently cooled to room temperature and the embossing stamp  330  is removed to provide an array of tapered cavities  340  within the polymer layer  310  separated by an array of banks  350 . In Step Three, a SAM such as 1H, 1H, 2H, 2H-perfluorodecanethiol (0.005 mol in ethanol) is applied to the gold surface. After inking a polydimethylsiloxane (PDMS) block  360  using the solution for about one minute, the PDMS stamp  360  is dried using nitrogen flow. Then, the stamp is brought into close contact with the structured sample surface for 30 seconds, and a hydrophobic SAM layer  370  is formed. To create better contact printing quality and improve the wetting contrast, the sample can be treated by oxygen plasma before the contact printing. Then TiO 2  colloid and dye  380  are printed by ink-jet printing into the cavities. 
     After drying, the device is completed by arranging a counter electrode (not shown) such as a Pt/ITO coated glass or PEN at a 20 μm distance having an electrolyte inserted into the device. As an example, the electrolyte is a redox electrolyte such as an iodine and potassium iodine mixture in acetonitrile. 
     In this embodiment a tapered embossing tool was used to improve the structured Au film quality. A standard embossing tool with vertical walls can be used as well. Because the cavities are filled by semiconductor, any small cracks or punch-through of the Au layer will not cause obvious damage of the device quality. 
     In another embodiment, a conductive polymer layer (such as polyaniline) can be introduced in between the metal layer  320  and the embossed polymer layer  310  to ensure conductivity. The conductive polymer layer can follow the cavity shape and can be difficult to be punch through. As the Au layer is 100 nm thick, the image can only be viewed through the second electrode, which is made transparent. This limitation can be overcome by selecting a transparent first electrode, or depositing a very thin metal (which is used only for touching SAM layer) on a transparent conductive polymer layer. 
     The foregoing description has been given by way of example only and a person skilled in the art will appreciate that modifications can be made without departing from the scope of the present invention. Other embodiments considered to be within the scope of the present invention include:
         (1) Alternative deposition techniques include doctor blading, printing (e.g. screen printing, offset printing, flexo printing, pad printing, and inkjet printing), evaporation, sputtering, chemical vapour deposition, spin-coating, dip and spray coating and electroplating.   (2) Alternative ways of surface treatment of a conducting layer include corona discharge treatments, UV-ozone treatments, chemical reaction, coating and vacuum deposition.   (3) Alternative materials for the SAM include polymer or small molecules with head group, such as silane, thiol etc. and tail groups, such as —NH 2  —COOH, —OH, —F, —CH3 etc.   (4) The conductive materials can be metals, organic or inorganic colloidal suspension, conductive polymer, such as a poly(3,4-ethylenedioxythiophene)-polystyrenesulphonic acid (PEDOT-PSS) water suspension and polyaniline.   (5) The fabrication process can be used for both “sheet-to-sheet” and “roll-to-roll” processes and the substrate can be both flexible and rigid, such as glass, poly(ethylenenaphthalate) (PEN), poly(ethyleneterepthalate) (PET), polycarbonates (PC), polyethersulphone (PES) and polyetheretherketon (PEKK).   (6) The embossing steps can be performed by thermally deforming a polymer or embossing a liquid polymer at room temperature and then curing the materials through thermal annealing or UV irradiation.   (7) A range of materials can form the embossing stamp  100 . The materials include semiconductors (Si and Ge), metals (Ni, Pt, W), alloys, ceramics, or polymers with a high glass transition temperature.   (8) Similar bank structures as obtained and described in accordance with the third embodiment of the present invention can be obtained by optical lithography, laser ablation, micro-embossing assisted by capillary flowing and selectively peeling-off film by detachment using a structured stump.   (9) The present invention is applicable to the manufacture of other electrochemical cells such as an Electrochromic Display Device (ECD). A typical ECD has a structure similar to that of a DSSC device as illustrated in  FIG. 1 . However, the functional dye layer  4  is replaced by an electrochromic material layer  4 . An ECD undergoes a reversible colour change when an electric current or voltage is applied across the device. The nanostructure type ECD comprises a molecular monolayer of electrochromic material, which is transparent in oxidised state and coloured in reduced state.