WO2009001343A2

WO2009001343A2 - Dry cell having a sintered cathode layer

Info

Publication number: WO2009001343A2
Application number: PCT/IL2008/000856
Authority: WO
Inventors: Eli Rosh Hodesh; Jonathan R. Goldstein; Barry Breen; Ilya Yakupov
Original assignee: 3Gsolar Ltd.
Priority date: 2007-06-24
Filing date: 2008-06-24
Publication date: 2008-12-31
Also published as: AU2009250946B2; AU2009250946A8; WO2009001343A3; AU2009250946A1

Abstract

A photovoltaic cell including: (a) a housing adapted to enclose the photovoltaic cell, including a transparent cell wall; (b) an electrolyte containing an iodine-based redox species, disposed within the cell wall; (c) an at least partially transparent conductive coating disposed on an interior surface of the cell wall; (d) an anode disposed on the conductive coating, including: (i) a porous film adapted to make intimate contact with the redox species, the porous film including chemically bonded titania nanoparticles, and (ii) a dye, absorbed on a surface of the porous film, the dye and the film adapted to convert photons to electrons; (e) a cathode, disposed within an interior surface of the housing, substantially opposite the anode, including (i) a conductive carbon layer, and (ii) a catalytic component, associated with the carbon layer and adapted to catalyze a redox reaction of the redox species, and to transfer electrons from the catalytic component to a current collection component of the cathode; the conductive carbon layer and the catalytic component disposed in electrolytic communication, via the electrolyte, with the porous film, the conductive carbon layer including (A) carbon black particles; (B) fine, typically spheroidal graphite particles; (C) expanded graphite particles, and (D) titania particles, the carbon black particles, the graphite particles, the expanded graphite particles, and the titania particles being intimately mixed, the titania particles of the conductive carbon layer being chemically bonded to the expanded graphite particles.

Description

Dye Cell Having a Sintered Cathode Layer

This application draws priority from U.S. Provisional Patent Application Serial No. 60/945,922, filed June 24, 2007, and from PCT Application No. IL2008/000671, filed May 15, 2008.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to photovoltaic dye cells for producing electricity directly from sunlight, and more particularly, to iodine-based dye cells having a sintered cathode layer.

The invention has particular relevance for solar cells of the dye-sensitized type (the so called DSSC or dye-sensitized solar cell) in which a high current density of operation at minimal ohmic loss is advantageous. Dye-sensitized photovoltaic cells for producing electricity from sunlight have been disclosed by U.S. Patent No. 5,350,644 to Graetzel, et al. U.S. Patent No. 5,350,644, which is incorporated by reference for all purposes as if fully set forth herein, teaches a photovoltaic cell having a light-transmitting, electrically conductive layer deposited on a glass plate or a transparent polymer sheet to which a series of titanium dioxide layers have been applied.

Following U.S. Patent No. 5,350,644, U.S. Patent No. 6,069,313 to Kay teaches a plurality of series-connected cell elements arranged as electrically separate, parallel, narrow elongated strips on a common transparent substrate. Each element includes a light facing anode comprising nanocrystalline titania, a carbon-based counter electrode (cathode), and an intermediate electrically insulating porous layer separating the anode from the cathode. The pores of the intermediate layer are at least partially filled with a liquid phase, ion-transferring electrolyte, following coating of the nanocrystalline titania with a light-sensitive dye. A current collecting layer of a tin oxide based, transparent, electrically-conducting material is situated between the transparent substrate and the anode. The anode and cathode of a given cell provide a direct current voltage when the anode is exposed to light, such that series assemblies of cells may readily be built up. The cathode of each succeeding element is connected with the inteπnediate conducting layer of the preceding anode element, over a gap separating the respective intermediate layers of these two elements. The series of cells (also known as a monolithic assembly of cells) is then sealed using an organic polymer, ensuring in particular that each individual strip cell is sealed from neighboring cells. Generally, dye cells have two electrodes separated by an electrolyte, with one electrode (the photoelectrode or photoanode) facing the sun or light source. Each electrode is supported on its own current collector, usually a sheet of conducting glass, which is glass coated on one side with a thin (-0.5 micrometer) transparent layer, usually based on electrically conductive tin oxide. The conducting glass sheets act as transparent walls of the dye cell.

A transparent polymer may be used in place of glass to support the tin oxide. The photoelectrode or photoanode includes a transparent porous layer about 10 micrometers thick (in contact with the tin oxide layer) based on titania, having a nanocrystalline characteristic particle size of 5-50 nm, applied by baking onto the conductive glass or conductive transparent polymer, and impregnated with a special dye. The baked-on titania layer is applied in dispersion form by any of various methods: doctor-blading, rolling, spraying, painting, electrophoresis, gravure printing, slit coating, screen printing or printing. The baking step giving highest cell performance is usually performed at a temperature of at least 450C, requiring the use of conducting glass rather than plastic for supporting the titania layer. Other processing procedures for the titania layer are feasible, such as reduced temperature baking, or pressing, usually with some sacrifice in cell efficiency. It is important to note that the titania is principally in contact with the tin oxide. The presence of other conductors (such as many metals, carbon and the like, on the photoanode), even if they are chemically inert to the electrolyte, can greatly increase recombination of charge carriers and provide a serious efficiency loss in the cell. Very few materials (amongst them tin oxide and titanium metal for example) are applicable to the photoanode that combine both chemical inertness to the electrolyte with relative freedom from recombination effects. For cells that are partially transparent, the other electrode in the cell (the counter electrode) includes a thin layer of catalyst (usually containing a few micrograms of platinum per sq. cm) on its respective sheet of tin oxide coated conductive glass or transparent plastic. If cell transparency is not required, the counter electrode can be opaque, for example, based on carbon or graphite advantageously catalyzed with trace platinum or another catalyst. The electrolyte in the cell is usually an organic solvent with a dissolved redox species. The electrolyte is typically acetonitrile or a higher molecular weight reduced volatility nitrile, with the redox species in early cells including dissolved iodine and potassium iodide-essentially potassium triiodide. Other solvents, salts and phases, for example ionic liquids with no vapor pressure, and even different redox species, may be used, however.

U.S. Patent No. 5,350,644 to Graetzel et al. discloses various dye cell chemistries, especially different dyes based on ruthenium complexes. Photons falling on the photoelectrode excite the dye (creating activated oxidized dye molecules), causing electrons to enter the conduction band of the titania and to flow (via an outer circuit having a load) to the counter electrode. There, the electrons reduce triiodide to iodide in the electrolyte, and the iodide is oxidized by the activated dye at the photoanode back to triiodide, leaving behind a deactivated dye molecule ready for the next photon. U.S. Patent No. 5,350,644 discloses that such dye cells can attain a solar-to-electric conversion efficiency of 10%.

The cells of U.S. Patent No. 5,350,644 to Graetzel, et al., are based on two sheets of conductive glass sealed with organic adhesive at the edges (the conductive glass projects beyond the adhesive on each side, allowing for current takeoff). These cells operate at a voltage of about 700 mV and a current density of 15 mA/sq. cm under peak solar illumination, with the counter electrode being the positive pole. It is asserted therein that since the materials and preparation methods are low cost and the titania layer can be prepared in large areas, such cells could potentially provide a good route to low-cost photovoltaic cells. It is further argued that there might be significant cost savings over classical single crystal or polycrystalline silicon cells and even more recent thin film photovoltaic cells, since these are all high cost and rely on expensive and often environmentally problematic raw materials, together with complex, costly, semiconductor industry processing equipment and production techniques. These drawbacks include the use of vacuum deposition and semiconductor doping methods, clean room protocols, use of toxic hydrides such as silane, phosphine, etc. as raw materials, and the use of toxic active layer materials containing cadmium, selenium or tellurium.

The ohmic loss via the conductive glass coated with tin oxide is a major problem of dye cells. The tin oxide coating is extremely thin, being limited in thickness usually to below one micrometer due to the need to maintain a high light transmittance through to the titania/dye layer of the photoanode. Moreover, tin oxide is only semiconductive and is difficult to bond to, such that the current takeoff is significantly limited by such a cell design to very small sized cells or strip cells with long narrow strips of active titania. It should be noted that active strip cells of this approach have certain technical disadvantages. For example, in the cells of Kay described above, the strips of titania are disadvantageously narrow (typically 6-8 mm wide), due to the ohmic loss restriction. This results in an excessive loss of active area between cells, due to the practical width of inert materials needed for intercell sealing. In any event, adequate sealing between adjacent cells so as to effectively prevent any intercell electrolyte migration remains a serious challenge.

Efforts have been made to increase the active area and breadth of cells by laying down parallel conducting strips on a conducting glass surface, thereby enabling a large- area, broad-cell construction. A photovoltaic cell having electrically conducting silver lines on spaced, glass support panes is disclosed by U.S. Patent No. 6,462,266 to Kurth, which is incorporated by reference for all purposes as if fully set forth herein. This disclosure has reduced ohmic loss with respect to the cell disclosed by U.S. Patent No. 5,350,644 to Graetzel, et al., because the silver lines are good conductors (e.g., silver paste screenprinted on and fired at 600⁰C), and because the overall effective thickness of conducting materials has been increased. However, the high process temperatures required for the silver and glaze compositions adversely affect the conductivity and strength properties of the glass. Glaze materials that can be processed at lower temperatures are available, but contain toxic heavy metals such as lead, which may be attacked by the electrolyte and poison the cathode catalyst. Furthermore, protection of silver conductors by a glass layer is inadequate over time, since pinholes cannot be entirely eliminated in the glaze, especially in mass production processes. In fact, exposed silver is spontaneously corroded by the iodine in cell electrolytes, adversely reducing the iodine inventory in the cell and irreversibly attacking the dye, and a similar deterioration occurs with silver lines protected by polymer. Of course, the additional width needed for protective glaze or polymer coatings over the silver lines further reduces the cell active area.

To date, there has been no real commercialization of photovoltaic dye cells, despite the great techno-economic potential thereof. The principal problems remaining include scale-up of cells to widths much above one centimeter — and areas much above 50 sq. cm - - due to excessive ohmic losses from the poorly conducting tin oxide layers on the glass or plastic, poor conductivity of carbon based cathode materials, long term stability of the usual silver based conductors in the cell, and difficulties of sealing the cells against long term dryout and performance degradation. Many approaches rely on costly in-house coating of the transparent substrate with conductive tin oxides rather than using a commercially available conductive glass having an acceptable cost such as FTO glass (glass coated with fluorine doped tin oxide).

A further problem in prior art dye cells and modules has been excessive surface area wasted in seals, protective layers and conducting paths on the sun-facing side of the cell or module. The active current-producing area in such cases is often less than 70% of the geometric area (footprint) of the cell or module, providing a poor effective efficiency from the available area. A yet further problem in prior art cells, especially if the counterelectrode is a separate element from the anode, is the relatively large spacing between anode and cathode — approaching 100 micrometers — which results in excessive ohmic loss from the electrolyte

U.S. Patent Application Publication No. 20050072458 to Goldstein, which is incorporated by reference for all purposes as if fully set forth herein, describes a large area, broad conductive glass or conductive plastic for a dye cell. The conductive glass or conductive plastic carries a set of conductors selected from materials intrinsically resistant to corrosion and to carrier recombination in the presence of the cell electrolyte, and onto this superior glass or plastic (having enhanced current collecting properties over plain conductive glass or plastic) the titania is deposited. By way of example, a conductive glass face is first grooved, giving a set of parallel spaced shallow grooves. Into each groove is placed a wire of a metal such as titanium, molybdenum, tungsten, chromium or their alloys (inert to corrosion and to carrier recombination under the operating conditions of the cell) and electrical conductivity between the wire and the tin oxide layer on each side of the groove is achieved using a heat-curable binder paste based on an inert ceramic adhesive (such as alumina) mixed with an inert, electrically conducting filler (such as titanium nitride). The paste fills the groove and overlaps on each side of the groove to make good electrical contact with the tin oxide layer there following curing. The wires exit from the cell from the groove extremities at the glass edges and may be sealed in and welded to a current-collecting strip. In a separate embodiment of this published application there is described a set of parallel strips of a metal or metal alloy having stability under cell operating conditions that is plated onto the conductive glass or conducting plastic surface. One example given of a plated metal is chromium. Current takeoff from the anode plate is again made from the side of the cell.

In a still further embodiment of this published application, the parallel conductors are inert strips or wires of titanium, molybdenum, tungsten, chromium or their alloys bonded directly to the conducting surface of the glass by means of an inert, electrically conducting ceramic adhesive.

U.S. Patent Application Publication No. 20050072458 extends the use of wires in a grooved conductive glass or of strips plated on conductive glass also for use in the counter electrode (cathode) of the cell. The glass plate, provided with wires bonded in grooves or with plated strips on the conductive surface, is used as a base for a broad cathode in the dye cell, and the conductivity-augmented plate is covered with a catalytic layer electroactive to iodine. Such a cathode, unfortunately, although fitted with adequate conducting means for current takeoff from a large area broad cell, necessarily includes a second layer of conducting glass in the cell, with associated cost, weight and thickness penalties. Broad dye cells of at least 10-15 cm per side are made possible, however. It should be evident from the above that when the counterelectrode is a separate element from the anode in the cell, the counterelectrode usually requires the use of a second glass support in the cell, or even worse, a second conductive glass in the cell. These greatly add to the cost, weight and thickness of the cell. In our unpublished PCT Application IL2008/000671 drawing priority from U.S.

Provisional Application No. 60/917,941, both of which are incorporated by reference for all purposes as if fully set forth herein, we disclosed a design for large area single dye cells in which the conductive glass of the photoanode is prepared with a parallel set of inert wires bonded into grooves using an inert ceramic conductive adhesive, or has plated onto its conductive surface a set of inert strip conductors for current takeoff. In one embodiment, based on an open sandwich design, a titania layer followed successively by a porous insulating spacer layer, and then a porous cathode layer (included typically of a catalytic carbon layer and a conducting carbon layer), are then sintered onto this enhanced conductivity support. Adequate current takeoff from the cathode is achieved by incorporating metal wires, mesh or perforated sheet at least partially embedded in the carbon. The titania is then coated with dye and the cell is filled with electrolyte by direct application of solutions to the porous cathode surface. The cell is then sealed using polymer means or using a glass sheet with edge sealing. Close spacing of anode and cathode may be achieved. In other embodiments for the large cell, a metal foil, itself catalyzed or carrying a catalyzed carbon layer, is used to close off the cell sandwich and take out the current from the cell. In yet other embodiments, the sintered-on spacer layer may be omitted and adequate spacing off from the titania is achieved using an edge- mounted gasket or a freestanding porous separator. Lowest cost cells, whether of the monolithic multicell type or the large area single cell type, will advantageously employ a carbon-based cathode. If possible, such cells are devoid of precious metal catalyst additions. As described in a paper by Graetzel in the Journal of the Electrochemical Society, VoI 153, p. A2256 (2006), such a carbon-based cathode element may be prepared from an aqueous dispersion of carbon black and colloidal nanocrystalline titania as binder. However, the paper refers only to doctor blading of carbon cathodes for very small area cells (area 0.36 sq. cm) and does not relate to screenprinting of larger area practically sized cells where areas in excess of 10 sq cm are required. Similar cathode formulations are described in U.S. Patent No. 6,069,313 to Kay, which is incorporated by reference for all purposes as if fully set forth herein. In this patent, particulate nanocrystalline titania is used as binder material, and additional conductive fillers to carbon black, such as graphite powder and doped tin oxide, optionally catalyzed with platinum, may be included). Furthermore, in patent document WO04109840 to Suzuki, nanocarbon structures such as carbon nanotubes and metal- bearing carbons are incorporated in the cathode. Such compositions may be inherently unsuitable for mass production cells prepared by sintering.

There is therefore a recognized need for, and it would be highly advantageous to have, improved iodine-based dye cells having a sintered cathode layer, and improved methods for producing such iodine-based dye cells.

SUMMARY OF THE INVENTION

According to the teachings of the present invention there is provided a photovoltaic cell for converting a light source into electricity, the cell including: (a) a housing adapted to enclose the photovoltaic cell, the housing including an at least partially transparent cell wall, the cell wall having an interior surface; (b) an electrolyte, disposed within the cell wall, the electrolyte containing an iodine-based redox species; (c) an at least partially transparent conductive coating disposed on the interior surface of the cell wall, within the photovoltaic cell; (d) an anode disposed on the conductive coating, the anode including: (i) a porous film adapted to make intimate contact with the redox species, the porous film including chemically bonded titanium dioxide nanoparticles, and (ii) a dye, absorbed on a surface of the porous film, the dye and the film adapted to convert photons to electrons; (e) a cathode disposed within an interior surface of the housing, the cathode disposed substantially opposite the anode, the cathode including: (i) a conductive carbon layer, and (ii) a catalytic component, associated with the carbon layer and adapted to catalyze a redox reaction of the redox species, the conductive carbon layer adapted to transfer electrons from the catalytic component to a current collection component of the cathode; the conductive carbon layer and the catalytic component disposed in electrolytic communication, via the electrolyte, with the porous film; the conductive carbon layer including (A) carbon black particles; (B) graphite powder; (C) expanded graphite particles, and (D) titanium dioxide particles, the titanium dioxide particles of the conductive carbon layer being chemically bonded to the expanded graphite particles.

According to another aspect of the present invention there is provided a method of producing a photovoltaic cell for converting a light source into electricity, the method including the steps of: (a) providing: (i) a housing adapted to enclose the photovoltaic cell, the housing including an at least partially transparent cell wall, the cell wall having an interior surface; (ii) an at least partially transparent conductive coating disposed on the interior surface of the cell wall, within the photovoltaic cell; (iii) an anode disposed on the conductive coating, the anode including a porous film adapted to make intimate contact with a redox species, the porous film including titanium dioxide nanoparticles, and (b) providing a cathode generally opposite the anode, the cathode including a sintered conductive carbon layer, the conductive carbon layer including: (i) carbon black particles; (ii) graphite powder; (iii) expanded graphite particles, and (iv) titanium dioxide particles, the titanium dioxide particles of the conductive carbon layer being chemically bonded to the expanded graphite particles.

According to yet another aspect of the present invention there is provided a method of producing a photovoltaic cell for converting a light source into electricity, the method including the steps of: (a) providing: (i) a housing adapted to enclose the photovoltaic cell, the housing including an at least partially transparent cell wall, the cell wall having an interior surface; (ii) an at least partially transparent conductive coating disposed on the interior surface of the cell wall, within the photovoltaic cell; (iii) an anode disposed on the conductive coating, the anode including a porous film adapted to make intimate contact with a redox species, the porous film including titanium dioxide nanoparticles; (b) disposing a conductive carbon paste opposite the anode, the paste including: (A) carbon black particles; (B) graphite powder; (C) expanded graphite particles, (D) at least one ceramic oxide precursor, the precursor selected from the group of precursors consisting of a titania precursor and a zirconia precursor, and (E) a liquid vehicle, the carbon black particles, the graphite powder, the expanded graphite particles, and the titanium dioxide particles being intimately mixed, and (c) sintering the conductive carbon paste at a temperature above 300C, to convert the at least one ceramic oxide precursor to ceramic oxide particles, and producing a conductive carbon cathodic layer from the conductive carbon paste.

According to yet another aspect of the present invention there is provided a method of producing a photovoltaic cell for converting a light source into electricity, the method including the steps of: (a) providing: (i) a housing adapted to enclose the photovoltaic cell, the housing including an at least partially transparent cell wall, the cell wall having an interior surface; (ii) an at least partially transparent conductive coating disposed on the interior surface of the cell wall, within the photovoltaic cell; (iii) an anode disposed on the conductive coating, the anode including a porous film adapted to make intimate contact with a redox species, the porous film including titanium dioxide nanoparticles; (b) providing a conductive carbon paste, the paste including: (A) carbon black particles; (B) graphite powder; (C) expanded graphite particles; (D) at least one ceramic oxide precursor, the precursor selected from the group of precursors consisting of a titania precursor and a zirconia precursor, and (E) a liquid vehicle, the carbon black particles, the graphite powder, the expanded graphite particles, and the titanium dioxide particles being intimately mixed; (c) disposing the conductive carbon paste on a matrix, and (d) sintering the conductive carbon paste at a temperature above 300C, to convert the at least one ceramic oxide precursor to ceramic oxide particles, and to produce a cathodic layer in which the ceramic oxide particles are sintered to the expanded graphite particles, and in which the ceramic oxide particles are sintered to the matrix.

According to further features in the described preferred embodiments, the conductive carbon layer is a sintered conductive carbon layer. According to still further features in the described preferred embodiments, the porous film is sintered to the conductive coating.

According to still further features in the described preferred embodiments, the carbon black particles, graphite powder, expanded graphite particles, and titanium dioxide particles are distributed in a substantially uniform manner within the sintered conductive carbon layer.

According to still further features in the described preferred embodiments, the conductive carbon layer is in a state of rigid attachment to the porous film.

According to still further features in the described preferred embodiments, the cell further includes: (f) a porous separator layer interdisposed between the porous film of the anode and the catalytic component of the cathode, the porous separator layer adapted to physically separate and electrically insulate between the porous film and the cathode.

According to still further features in the described preferred embodiments, the porous separator layer includes a spacing element that is physically distinct with respect to the porous film of the anode.

According to still further features in the described preferred embodiments, the porous separator layer includes a spacing element that is chemically bonded to the porous film of the anode.

According to still further features in the described preferred embodiments, the porous separator layer includes a spacing element that is sintered to the porous film of the anode.

According to still further features in the described preferred embodiments, the spacing element is selected from the group of structural elements consisting of glass fiber and microporous polymer. According to still further features in the described preferred embodiments, the chemically bonded titanium dioxide nanoparticles include sintered titanium dioxide nanoparticles.

According to still further features in the described preferred embodiments, the conductive carbon layer is in a state of rigid attachment with respect to the porous separator layer.

According to still further features in the described preferred embodiments, the conductive carbon layer is chemically bonded to the porous separator layer.

According to still further features in the described preferred embodiments, the conductive carbon layer is sintered to the porous separator layer. According to still further features in the described preferred embodiments, the cell further includes: (f) at least one metal strip or wire, electrically associated with the anode and with the conductive coating, the strip or wire having sufficient thickness to foπn a protrusion protruding above a plane of the porous film by at least 50 micrometers. According to still further features in the described preferred embodiments, the at least one metal strip or wire is bonded to the transparent conductive coating by a conductive binding material.

According to still further features in the described preferred embodiments, the conductive binding material includes a ceramic adhesive. According to still further features in the described preferred embodiments, the conductive binding material includes titanium nitride.

According to still further features in the described preferred embodiments, the at least one metal strip or wire is a plurality of wires, and the protrusion is a plurality of protrusions. According to still further features in the described preferred embodiments, the conductive carbon layer further includes: (E) zirconium oxide particles, preferably distributed in a substantially uniform manner with the carbon black particles, the graphite powder, the expanded graphite particles, and the titanium dioxide particles.

According to still further features in the described preferred embodiments, the carbon black and the expanded graphite have a weight ratio in a range of 1.5: 1 to 1 : 1.5.

According to still further features in the described preferred embodiments, the total carbon content of the conductive carbon layer includes the graphite powder, the carbon black particles, and the expanded graphite particles, and the total carbon content and the titanium dioxide particles have a weight ratio in a range of 1.2: 1 to 3.5:1. According to still further features in the described preferred embodiments, the graphite powder has a weight ratio with respect to the sum of the carbon black particles and the expanded graphite particles, the weight ratio falling in a range of 1.5: 1 to 3.5: 1.

According to still further features in the described preferred embodiments, the graphite powder has a weight ratio with respect to a sum of the carbon black particles, the expanded graphite particles, and the zirconium oxide particles, the weight ratio falling in a range of 1.2: 1 to 3: 1.

According to still further features in the described preferred embodiments, the total carbon content of the conductive carbon layer includes the graphite powder, the carbon black particles, and the expanded graphite particles, and the weight ratio of the total carbon content and the zirconium oxide particles to the titanium dioxide particles falls in a range of 1.5: 1 to 4: 1.

According to still further features in the described preferred embodiments, the width (i.e., the narrow dimension of the face) of the conductive carbon layer is at least 5 cm.

According to still further features in the described preferred embodiments, the width of the conductive carbon layer is at least 8 cm.

According to still further features in the described preferred embodiments, the width of the conductive carbon layer is at least 10 cm. According to still further features in the described preferred embodiments, the width of the conductive carbon layer is at least 15 cm.

According to still further features in the described preferred embodiments, the expanded graphite particles have a long dimension of at least 15 micrometers, on average.

According to still further features in the described preferred embodiments, the expanded graphite particles have a long dimension of at least 25 micrometers, on average.

According to still further features in the described preferred embodiments, the expanded graphite particles have a long dimension of at least 40 micrometers, on average.

According to still further features in the described preferred embodiments, the expanded graphite particles have a long dimension of at least 60 micrometers, on average. According to still further features in the described preferred embodiments, the titania particles have an average primary particle size below 50 nm.

According to still further features in the described preferred embodiments, the titania particles have an average primary particle size below 30 nm.

According to still further features in the described preferred embodiments, the method further includes, prior to providing the cathode having the sintered conductive carbon layer, (c) disposing a conductive carbon paste opposite the anode, the paste including: (i) carbon black particles; (ii) graphite powder; (iii) expanded graphite particles,

(iv) at least one ceramic oxide precursor, the precursor selected from the group of precursors consisting of a titania precursor and a zirconia precursor, and (v) a liquid vehicle, wherein the sintered conductive carbon layer is produced by sintering the conductive carbon paste at a temperature above 300C, to convert the at least one ceramic oxide precursor to sintered ceramic oxide particles. According to still further features in the described preferred embodiments, the disposing of the conductive carbon paste opposite the anode is performed by screenprinting the paste.

According to still further features in the described preferred embodiments, the method further includes the step of: (c) disposing a porous, electrically insulative separator on the anode, to electrically insulate between the anode and the cathode.

According to still further features in the described preferred embodiments, the method further includes the step of: (d) disposing a porous, electrically insulative separator on the anode, to electrically insulate between the anode and the cathode, wherein the disposing of the conductive carbon paste is performed directly on a surface of the porous, electrically insulative separator.

According to still further features in the described preferred embodiments, the ceramic oxide precursor includes a titanium or zirconium chelate.

According to still further features in the described preferred embodiments, the ceramic oxide precursor is a titania precursor.

According to still further features in the described preferred embodiments, the titania precursor includes an organic titanate.

According to still further features in the described preferred embodiments, the titania precursor includes a compound selected from the group of titania precursors consisting of organic titanium chelates, aqueous titanium chelates, organic titanates and ortho-titanate esters.

According to still further features in the described preferred embodiments, the titania precursor includes a titanium acetylacetonate.

According to still further features in the described preferred embodiments, the liquid vehicle includes an alcohol.

According to still further features in the described preferred embodiments, the sintering of the conductive carbon paste to convert the ceramic oxide precursor to sintered ceramic oxide articles is performed in an air-containing environment, the ceramic oxide precursor includes a fumeless precursor that reacts in the air-containing environment in a substantially non-fuming fashion.

According to still further features in the described preferred embodiments, the cathode includes a support matrix structurally supporting the conductive carbon layer and sintered thereto. According to still further features in the described preferred embodiments, the conductive carbon layer includes: (v) zirconia particles, intimately dispersed within the conductive carbon layer.

According to still further features in the described preferred embodiments, the conductive carbon layer includes: (vi) zirconia particles, intimately dispersed within the conductive carbon paste.

According to still further features in the described preferred embodiments, the method further includes the step of: (d) disposed within the cell wall, an electrolyte, the electrolyte containing an iodine-based redox species. According to still further features in the described preferred embodiments, the screenprinting of the paste is performed over a cell width of at least 5 cm, and preferably, at least 10 or at least 15 cm.

According to still further features in the described preferred embodiments, the sintered conductive carbon layer is produced by sintering the conductive carbon paste at a temperature above 350C.

According to still further features in the described preferred embodiments, the sintered conductive carbon layer is produced by sintering the conductive carbon paste at a temperature above 400C.

According to still further features in the described preferred embodiments, the matrix is selected from a group of matrices consisting of glass fiber mat, carbon fiber mat, zirconia felt, titania felt and carbon foam.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. Throughout the drawings, like-referenced characters are used to designate like elements.

In the drawings:

FIG. 1 is a schematic cross-sectional view of an exemplary photovoltaic cell having a biplate construction and a sintered cathode layer;

FIG. 2 is a schematic cross-sectional view of an exemplary embodiment of another inventive photovoltaic cell having a monolithic construction and a sintered cathode layer;

FIG. 3 is a schematic cross-sectional representation of a porous carbon paste layer following sintering, according to the present invention, and FIG. 4 is a schematic cross-sectional representation of a porous carbon paste layer following sintering, according to the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principles and operation of the photovoltaic dye cells according to the present invention may be better understood with reference to the drawings and the accompanying description.

Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

Figure 1 is a schematic cross-sectional view of an exemplary photovoltaic cell 600 having a biplate construction in which the anode and cathode components in the cell build are physically distinct and not bonded together. The photoanode of cell 600 includes a porous nanocrystalline titania layer or film 138. Porous titania layer 138 has an extremely thin layer of dye (typically a monolayer of dye) absorbed on a surface of the porous titania layer. In the photoanode of cell 600, which includes porous titania layer 138 having the layer of dye, the dye and film are adapted to convert photons to electrons.

A photoanode support glass 30 with transparent conductor layer 32 has spaced, preferably parallel wires or strips such as wire 110 bonded in place on the surface of the conductive glass by a conducting adhesive such as an inert conducting ceramic adhesive 115. Porous titania layer 138 is bonded to transparent conductor layer 32 in most of the area between adjacent wires 110. Both the titania and the conducting adhesive bonding the wires may be optionally covered by an electrically insulating layer, for example, consisting of, consisting essentially of, or including zirconia, for additional protection against short-circuiting.

A porous, laid-on separator 125, such as a glass fiber veil or a microporous polymer membrane, may electrically insulate the photoanode from a counterelectrode 192. Counterelectrode 192 may include a plurality of strips, each strip including a conducting carbon layer 185 and catalytic carbon layer 190, which may alternatively form a single layer. Conducting carbon layer 185 and/or catalytic carbon layer 190 may include an inert matrix including a porous mat, felt or foam based on glass fiber, carbon or a ceramic oxide such as zirconia or titania. Porous separator 125 may be extremely thin and porous (preferably having a thickness of less than 50 micrometers in an uncompressed state), and of a material that is chemically stable in the cell (for example: polyethylene, PET, PPS, PEN, fluoropolymer or glass fiber). Cathodic current collector such as collector sheet 310 may be made of, consist essentially of, or include, a metal sheet selected from the cathode metals and alloys provided hereinabove. Collector sheet 310 is adapted to firmly press against counterelectrode 192. In one embodiment not requiring counterelectrode strips, the metal sheet of collector sheet 310 can be directly catalyzed with platinum or a platinum substitute, and may be structured (by corrugation or machining, for example) with periodically spaced projecting areas (not shown) to ensure close proximity (via the porous membrane) of catalyst to titania between the wires of the photoanode. Referring again to Figure 1, a plurality of strips, each including conducting carbon

185 and catalytic carbon layer 190, may be sintered or otherwise bonded to collector sheet 310. In the assembled cell 600, counterelectrode 192 is disposed substantially opposite the titania printings, and may be of pre-designed thickness, such that there is a relatively close proximity (across the porous separator) of the catalyzed carbon layer and the titania layer between the wires after cell closure and sealing. Following dyeing of the titania, the two halves of the biplate cell are juxtaposed with the porous separator between them, metal collector sheet 310 is hot sealed to the glass anode base-plate edges using, by way of example, an inner polymer sealing layer or seal 165 and an outer epoxy sealing layer or seal 170. After checking that cell 600 is free from short circuiting, cell 600 is filled with electrolyte, one possibility being adding electrolyte to the cell via fill holes and later sealing off the fill holes with polymer. Current take-off from collector sheet 310 out of the cell may be enabled by a projecting strip or a welded-on element.

Together, photoanode support glass 30 and collector sheet 310 form a housing substantially enclosing cell 600.

Figure 2 is a schematic cross-sectional view of an exemplary embodiment of another inventive photovoltaic cell 800 having a cathodic current collector including, or consisting essentially of, a conductive, chemically inert mesh or foil 195, preferably disposed above, and physically contacting, counterelectrode 320. In this embodiment, counterelectrode 320 is sintered onto electrically insulating layer 220, typically zirconia. Electrically insulating layer 220 typically has a relatively uniform thickness in a range of 2-15 micrometers. For configurations in which counterelectrode 320 includes more than one layer, e.g., a conductive layer and a catalytic layer, the catalytic layer is disposed towards electrically insulating layer 220. A photoanode support glass 30 coated with substantially transparent conductor layer 32 may have spaced, preferably parallel wires 110 bonded in place on the surface of the conductive glass by inert conducting ceramic adhesive 115. Between wires and at the cell edges is a sintered nanoporous titania layer 138, typically 10-15 micrometers thick, which is covered with electrically insulating layer 220, e.g., including zirconia. Since the wires may typically have a diameter of at least 100 micrometers, and the combined height of the titania and zirconia layers is typically below about 30 micrometers, the wires project substantially above the titania layer and may be covered also by an electrically insulating layer 225, for example including zirconia, to avoid short circuiting to counterelectrode 320. The sealing arrangement of cell 800 may be substantially identical to the arrangement provided in Figure 1 and described hereinabove.

The superior current collection performance of conductive mesh or foil 195 obviates the need for sealing the cathodic arrangement with expensive conductive glass such as FTO glass. Instead, cell 800 may be sealed on top by a sheet such as ordinary, inexpensive glass sheet 190, disposed above mesh or foil 195, and may also be adapted to provide the requisite structural support for mesh or foil 195 as well as the requisite compressive force against mesh or foil 195.

Conductive mesh or foil 195 may pass sealably out through seals 165, 170 of photovoltaic cell 800. Conductive mesh or foil 195 may include, or consist essentially of, a metal mesh or foil.

Conductive mesh or foil 195 may include, or consist essentially of, a graphite mesh or foil, or a carbon mesh or foil. Composite cathodic current collectors including graphite and/or carbon and/or metal meshes or foils may also be practical. It may be particularly advantageous to use a graphite or carbon foil to cover the central area of photovoltaic cell 800, and to boost current takeoff near the edges of cell 800 by electrically connecting the edges of the carbon or graphite foil to a metal mesh and/or foil disposed near a perimeter of cell 800 and passing sealably out through seals 165, 170 of cell 800. As mentioned hereinabove, the prior art does not provide carbon pastes that can be successfully applied by screenprinting to large area cells of the monolithic or single cell design with cell areas significantly above 10 sq. cm. Screenprinting, rather than doctor blading, is the application method of choice for industrial production of broad, large-area dye cells. However, when water based formulations are attempted for screenprinting of the carbon for dye cells there emerge several obstacles. Firstly, the prior art water-based carbon pastes tend to be quick drying and can clog the screens used in production scale screenprinting. Certain additives may ease this situation somewhat.

A more critical problem, however, is that after sintering, these pastes have insufficient electrical conductivity for practically useful printing thicknesses. Even more problematic is the fact that after screenprinting and sintering - and even with application of multiple layers — the layers have insufficient mechanical strength (bonding within the layer itself and to the substrate beneath) and can delaminate. Often, cracks and holes are evident in the porous structure and carbon particles may dislodge or delaminate from the layer, in some cases shorting out the cell between the conducting glass of the anode and the counterelectrode.

The insufficient mechanical strength is principally a result of the incorporation of particulate titania intrinsically as the binder, since this material does not sinter strongly enough to the carbon materials in the paste nor to the substrate beneath.

In the present invention, a titania precursor is used (instead of titania powder) which, during the usual sintering in air, provides titania centers in situ throughout the layer and forms stronger bonds with the carbon materials used and, in some preferred embodiments, with the substrate beneath (usually a spacer layer of porous zirconia). We have further discovered that many precursors that oxidize acceptably to titania on sintering in air, for example the titanium alkoxides, appear to be unsuitable for paste formulation, because they fume strongly in air and are difficult to work with. A non- limiting example of a benign, suitable precursor is the family of organic titanates supplied by DuPont under the trade name Tyzor™. These organic titanates are available in the form of organic chelates, aqueous chelates, and ortho-titanate esters. These precursors contain about 15 wt% of titania equivalent, and oxidation to titania is usually complete on heating in air at above 300C for 30 minutes. We have found particularly useful the Tyzor™ grade AA-75, which is based on titanium acetylacetonates in an alcoholic solvent. However, other grades of Tyzor™ are also applicable. The parallel zirconium chelates may also be used (which sinter in situ to zirconia), but these are more expensive than their titanium analogs.

We have further discovered that the conductivity and strength of the conductive carbon layer may be appreciably improved by the addition of a material known as expanded graphite to the paste. Prior art carbon pastes for the dye cell cathode (which were not conducting or robust enough) normally consist of high surface area conductive grade carbon black (at least 150 sq. meter per gram) with typical particle size of up to 50 nanometers, together with graphite powder having an average particle diameter of several micrometers as the main additive for boosting conductivity.

Expanded graphite, which is graphite that has been chemically exfoliated to give large, flat, anisometric, two-dimensional platelets of particle size selectable from about 20-100 micrometers and having a surface area of about 10-30 sq. meters per gram, is a much more effective conductivity booster and also provides improved anchoring and mechanical stability for the layer. The carbon, graphite and titania precursor components are conveniently dispersed in a printing vehicle such as terpineol or ethyl hexanol. The paste may be made more viscous and printable, giving a print with improved green strength, by the incorporation into the terpineol of a heat fugitive thickening agent such as ethyl cellulose. Additional conducting filler powders that are chemically stable in the cell working conditions, for example doped tin oxides, titanium nitride and metal powders or metal coated carbons, may be added to supplement the carbons. The various carbons (such as the carbon black) can also be optionally catalyzed, for example, with platinum, or alternatively a special thin layer (few micrometers) of catalyzed carbon may be printed adjacent to the titania or zirconia layers, situated underneath a conducting carbon layer that is substantially free of precious metal catalyst. For such thin layers, the catalyzed carbon paste will advantageously be made up containing a fine zirconia powder (typically having an average particle size of about 0.5 micrometers), which enhances the adhesion of the catalytic layer. Similarly, non-conducting bulking materials such as glass spheres (e.g., having a diameter of 40-80 micrometers) may be included in the paste in order to facilitate printing of thick adhesive carbon layers of thickness 100 micrometers and above. A schematic cut away representation of an inventive sintered porous carbon layer

100 is shown in Figure 3, for typical layer thicknesses of 50-100 micrometers. Porous carbon layer 100 includes at least three different types of carbon-based materials: graphite powder particles such as graphite particle 1, carbon black particles such as carbon black particle 2, and large leaf-like platelets of expanded graphite such as expanded graphite particle 3. The various carbons (such as the carbon black) may optionally be catalyzed. Porous carbon layer 100 further includes titania particles, such as titania particle 4 which has been freshly generated in-situ. Without wishing to be bound by theory, we believe that such freshly generated titania strongly bonds the different materials together and to the substrate below, in those embodiments in which carbon layer 100 is sintered thereto (such as electrically insulating layer 220, shown in Figure 2). Figure 3 shows that there is good electrical continuity of the carbon particles through the sintered paste, conferring a high conductivity. We further believe that the expanded graphite particles provide a conduit for removing the cathodic current at minimal ohmic loss.

Some titania particles may bond directly to the graphite particles, others may bond directly to the carbon black particles. We have discovered that in the photovoltaic cell of the present invention, titania particles such as titania particles 4a may bond directly to the expanded graphite platelets such as expanded graphite particle 3, or to carbon black particles 2 or to graphite powder particles 1. In sintered porous carbon layer 100, such bonds are chemical bonds, which provide for a more robust, mechanically solid carbon layer.

In a preferred embodiment, the conductive paste includes zirconia particles and/or a zirconia precursor. Figure 3 further provides a schematic representation of zirconia particles 5 disposed within sintered porous carbon layer 100. The zirconia particles may be sintered to particles 1,2,3,4, providing for a more robust, mechanically solid carbon layer.

In sharp contrast to the microstructure of inventive sintered carbon layer 100, an exemplary, schematic cross-sectional cut away illustration of a prior art sintered porous carbon layer 105 is provided in Figure 4. As may be seen, spheroidal graphite particles 1 and carbon black particles 2 do not provide an adequate electrically conductive network, while titania particles 14, which were not generated in-situ, are somewhat isolated and do not confer much mechanical strength to the layer 105, nor can they provide effective bonding to the substrate below, in those embodiments in which carbon layer 105 is sintered thereto (such as electrically insulating layer 220, shown in Figure 2). Moreover, the pastes used to produce the prior art sintered porous carbon layers may be generally difficult to screenprint, may be mechanically weak, and may not have adequate conductivity for dye cells, and more particularly, for broad, large-area dye cells.

The graphite powder used to produce sintered carbon layer 100 may typically include spheroidal particles having an average particle size of at least 2 micrometers, and more typically, at least 5 micrometers, up to a maximum average particle size of 10 to 15 micrometers. The surface area of the graphite particles is typically 15-40 m²/g, and more typically, 20-30 m²/g.

The expanded graphite particles used to produce sintered carbon layer 100 may be typically anisotropic flakes having a long dimension of at least 15 micrometers, more typically at least 40 micrometers, and yet more typically at least 60 micrometers, on average. The surface area of the expanded graphite particles is typically 5-50 m /g, and more typically, 25-35 m7g.

The carbon black particles used to produce sintered carbon layer 100 may be typically disposed in chainlike clusters of carbon, in which the individual particles may have an average primary particle size of 5-100 nm, and more typically, 10-75 nm. The surface area of the carbon black particles is typically 50-2000 m²/g, more typically, 100-

600 m²/g, and most typically, 150-300 m²/g.

The titania particles in sintered carbon layer 100 may be particles having an average primary particle size of at least 10 nm, and more typically, at least 20nm to about 30nm. The surface area of the titania particles is typically 20-130 m /g, and more typically, 50-90 m²/g.

The zirconia particles in sintered carbon layer 100 may be spheroidal particles, the zirconia particles having an average particle size of at least 0.3 micrometers, and more typically, at least 0.5 micrometers, up to an upper bound of typically one micrometer. The surface area of the zirconia particles is typically 5-40 m²/g, and more typically, 10-20 m /g. When formed in-situ, the zirconia particles are typically much smaller.

In reducing sintered carbon layer 100 of the present invention to practice, we have discovered that various weight ratios may be of particular importance. The weight ratio of spheroidal graphite to the sum of carbon black and expanded graphite may be preferably in the range of 1.5: 1 to 3.5: 1.

The weight ratio of carbon black particles to expanded graphite particles may be preferably in the range of 1.5: 1 to 1 : 1.5.

The weight ratio of the total carbon content of sintered carbon layer 100 (including the graphite powder, carbon black particles, and expanded graphite particles) to titanium dioxide particles, may be preferably in the range of 1.2: 1 to 3.5:1.

In those preferred embodiments in which zirconia particles are present in sintered carbon layer 100, the weight ratio of carbon black particles to expanded graphite particles to zirconia particles may be preferably in the range of 1.5: 1 :0.5 to 1 : 1.5: 1. The weight ratio of the total carbon content and zirconia content of sintered carbon layer 100 to the titania content may be preferably in the range of 1.5: 1 to 4: 1.

The weight ratio of the typically spheroidal graphite particles to a sum of the carbon black particles, expanded graphite particles, and zirconia particles may be preferably in the range of 1.2: 1 to 3: 1. As used herein in the specification and in the claims section that follows, the term

"precursor", with respect to titania or zirconia, refers to a titanium-containing or zirconium-containing compound that, upon heating above at least 300C, reacts to produce titania or zirconia.

As used herein in the specification and in the claims section that follows, the term "specific surface area", and the like, refers to the specific surface area measured by a standard BET analysis using a nitrogen medium.

As used herein in the specification and in the claims section that follows, the terms "titania" and "titanium dioxide", are used interchangeably, and the terms "zirconia" and "zirconium dioxide", are used interchangeably.

EXAMPLES

Reference is now made to the following examples, which together with the above descriptions, illustrate the invention in a non-limiting fashion.

Preparation of a dye cell carbon cathode according to the prior art is described in the following example:

Example 1: Preparation of a dye ceil carbon cathode according to the prior art

Conductive carbon black (Cabot type Vulcan XC72R, 5 gm), spherical graphite powder (Timcal, type KS4, 20 gm), and titania nanocrystalline powder (Degussa type P 90, particle size 14 nm, 4 gm) were dry blended in an agate ball mill for 30 minutes. A quantity of water (80 gm) was then added to the ball mill containing the carbon, graphite and the titania powder and milling was continued overnight. The resulting carbon paste (as a cathode paste) was evaluated for fabrication of small area dye cells (typically below 1-5 sq. cm). The paste could be doctor-bladed and sintered onto conductive glass, or onto conductive glass with a lower sintered layer of porous titania covered by an upper sintered layer of porous zirconia. The carbon layer was adherent for these small cells, with a conductivity of 10 ohms per square for a thickness of 50 micrometers. On dye and electrolyte application and sealing of cells, a good efficiency of 8% under one sun illumination (100 mW per sq. cm) was achieved. However, when screenprinting was attempted with the paste for larger area cells typically above 10 sq. cm. with a view to industrial scale up, the paste tended to clog the printing screens, and the print following sintering had inadequate mechanical strength and conductivity. Particularly problematic was poor adhesion to the substrate, delamination and formation of cracks and holes over the surface.

Preparation of a dye cell carbon cathode according to the present invention is described in the following example:

Example 2: Preparation of a screenprintable carbon dye cell cathode

Spherical graphite powder (Timcal, type KS4, 20gm), carbon furnace black (Cabot, type Vulcan XC72R, 5gm) and expanded graphite (Timcal, type BNB90, 2.5 gm) were dry blended in an agate ball mill for 30 minutes. A titania precursor (Dupont, Tyzor™ type AA-75, 30gm) was stirred with terpineol (Aldrich, 50 gm) that contained 10wt% dissolved ethyl cellulose (Dow, ethocell type standard industrial 45). This liquid mixture was then added to the ball mill containing the carbon, spherical graphite powder, and expanded graphite, and milling was continued overnight. The resulting carbon paste was screenprinted in a single printing using a plastic mesh onto a sintered porous zirconia spacer upper layer, itself part of a conducting glass carrying a sintered-on layer of nanocrystalline titania (15 micrometers thick) covered by the porous sintered zirconia layer and appropriate for dye cell fabrication for cell areas in excess of 10 sq. cm. The wet thickness of the carbon print was 100 micrometers. The carbon print was dried at 150C for 10 minutes. The print was then sintered in air at 450C for 30 minutes, giving a sintered thickness of 50 micrometers. In order to achieve the desired conductivity, a second printing of carbon paste, drying and sintering was carried out, and the final sintered carbon thickness was 50 micrometers. Adherence of the carbon layer to the underlying zirconia layer was excellent, there was good integrity of the layer with no flaking off, and there were no cracks or holes. The electrical conductivity of the layer was 10 ohms per square at this thickness. Since the conducting glass used to support the titania layer had a similar conductivity of 10 ohms per square there was good matching of photoanode and counter electrode conductivities, one of the requisites for high dye cell efficiencies. The carbon layer was quite porous and allowed fast transfer of dye and electrolyte printings to the Jitania layer below. The dye cell, after dye staining of the titania, electrolyte addition and sealing, gave 7% efficiency under one sun illumination of 10OmW per sq. cm.

Example 3: Preparation of a screenprintable carbon dye cell cathode Spherical graphite powder (Timcal, type KS4, 20gm), carbon furnace black

(Cabot, type Vulcan XC72R, 5gm), expanded graphite (Timcal, type BNB90, 5 gm) and zirconium dioxide (IAM, type 40R-080, 3gm) were dry blended in an agate ball mill containing agate balls for 30 minutes. A titania precursor (Dupont, Tyzor™ type AA-75, 60gm) was stirred with terpineol (Aldrich, 25gm) that contained 10wt% dissolved ethyl cellulose (Dow, ethocell type standard industrial 45). This liquid mixture was then added to the ball mill containing the carbon, spherical graphite powder, and expanded graphite, and milling was continued overnight. The resulting carbon paste was screenprinted in a single printing using a plastic mesh onto a sintered porous zirconia spacer upper layer, itself part of a conducting glass carrying a sintered-on layer of nanocrystalline titania (15 micrometers thick) covered by the porous sintered zirconia layer and appropriate for dye cell fabrication for cell areas in excess of 10 sq. cm. The wet thickness of the carbon print was 100 micrometers. The carbon print was dried at 150C for 10 minutes. The print was then sintered in air at 450C for 30 minutes, giving a sintered thickness of 50 micrometers. In order to achieve the desired conductivity, a second printing of carbon paste, drying and sintering was carried out, and the final sintered carbon thickness was 50 micrometers. Adherence of the carbon layer to the underlying zirconia layer was excellent, there was good integrity of the layer with no flaking off, and there were no cracks or holes. The electrical conductivity of the layer was 10 ohms per square at this thickness. Since the conducting glass used to support the titania layer had a similar conductivity of 10 ohms per square there was good matching of photoanode and counter electrode conductivities, one of the requisites for high dye cell efficiencies. The carbon layer was quite porous and allowed fast transfer of dye and electrolyte printings to the titania layer below. The dye cell, after dye staining of the titania, electrolyte addition and sealing, gave 7% efficiency under one sun illumination of 10OmW per sq. cm.

Preparation of a dye cell carbon cathode in a single printing according to the present invention is described in the following example:

Example 4:

Preparation of a screenprintable carbon dye cell cathode in a single printing

Spherical graphite powder (Timcal, type KS4, 21gm), carbon furnace black (Cabot, type Vulcan XC72R, 4gm) and expanded graphite (Timcal, type BNB90, 3 gm) were dry blended in an agate ball mill for 30 minutes. A titania precursor (Dupont, Tyzor™ type AA-75, 30gm) was stirred with terpineol (Aldrich, 50 gm) containing 10 wt% dissolved ethyl cellulose (Dow, ethocell, type standard industrial 45). This liquid mixture was then added to the ball mill containing the carbon, spherical graphite powder, and expanded graphite, and milling was continued overnight. The resulting carbon paste was screenprinted using a plastic mesh onto a sintered porous zirconia spacer upper layer, itself part of a conducting glass carrying a sintered-on layer of nanocrystalline titania (15 micrometers thick) covered by the porous sintered zirconia layer and appropriate for dye cell fabrication for cell areas in excess of 10 sq. cm. The wet thickness of the carbon print was 100 micrometers. The carbon print was dried at 150C for 10 minutes. The print was then sintered in air at 450C for 30 minutes, giving a sintered thickness of 50 micrometers. Adherence of the carbon layer to the underlying zirconia layer was excellent, there was good integrity of the layer with no flaking off, and there were no cracks or holes. The electrical conductivity of the layer was 10 ohms per sq. at this thickness, similar to that of the conductive glass supporting the titania layer and adequate for good dye cell performance. The carbon layer was quite porous and allowed fast transfer of dye and electrolyte printings to the titania layer below. The dye cell, after dye staining of the titania, electrolyte addition and sealing, gave 7% efficiency under one sun illumination of 100m W per sq. cm. Preparation of a dye cell catalyzed carbon cathode in a single printing according to the present invention is described in the following example:

Example 5:

Preparation of a screenprintable catalyzed carbon dye cell cathode in a single printing

Spherical graphite powder (Timcal, type KS4, 17gm), carbon furnace black (Cabot, type Vulcan XC72R, 4gm), carbon furnace black catalyzed with platinum (Cabot, type Vulcan XC72R containing 1% platinum, 4gm), and expanded graphite (Timcal, type BNB90, 3 gm) were dry blended in an agate ball mill for 30 minutes. A titania precursor (Dupont, Tyzor™ type AA-75, 80gm) was stirred with terpineol (Aldrich, 50 gm) that contained 10 wt% dissolved ethyl cellulose (Dow, ethocell, type standard industrial 45). This liquid mixture was then added to the ball mill containing the carbon materials, spherical graphite powder, and expanded graphite, and milling was continued overnight. The resulting carbon paste was screenprinted using a plastic mesh onto a sintered porous zirconia spacer upper layer, itself part of a conducting glass carrying a sintered-on layer of nanocrystalline titania (15 micrometers thick) covered by the porous sintered zirconia layer and appropriate for dye cell fabrication for cell areas in excess of 10 sq. cm. The wet thickness of the carbon print was 100 micrometers. The carbon print was dried at 150C for 10 minutes. The print was then sintered in air at 450 C for 30 minutes giving a sintered thickness of 50 micrometers. Adherence of the carbon layer to the underlying zirconia layer was excellent, there was good integrity of the layer with no flaking off, and there were no cracks or holes. The electrical conductivity of the layer was 10 ohms per square at this thickness, similar to that of the conductive glass supporting the titania layer and adequate for good dye cell performance. The carbon layer was quite porous and allowed fast transfer of dye and electrolyte printings to the titania layer below. The dye cell, after dye staining of the titania, electrolyte addition and sealing, gave 8% efficiency under one sun illumination of 100m W per sq. cm.

Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations. All publications and patents mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication or patent was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.

Claims

WHAT IS CLAIMED IS:

1. A photovoltaic cell for converting a light source into electricity, the cell comprising:

(a) a housing adapted to enclose the photovoltaic cell, said housing including an at least partially transparent cell wall, said cell wall having an interior surface;

(b) an electrolyte, disposed within said cell wall, said electrolyte containing an iodine-based redox species;

(c) an at least partially transparent conductive coating disposed on said interior surface of said cell wall, within the photovoltaic cell;

(d) an anode disposed on said conductive coating, said anode including:

(i) a porous film adapted to make intimate contact with said redox species, said porous film including chemically bonded titanium dioxide nanoparticles, and

(ii) a dye, absorbed on a surface of said porous film, said dye and said film adapted to convert photons to electrons;

(e) a cathode disposed within an interior surface of said housing, said cathode disposed substantially opposite said anode, said cathode including:

(i) a conductive carbon layer, and

(ii) a catalytic component, associated with said carbon layer and adapted to catalyze a redox reaction of said redox species, said conductive carbon layer adapted to transfer electrons from said catalytic component to a current collection component of said cathode; said conductive carbon layer and said catalytic component disposed in electrolytic communication, via said electrolyte, with said porous film; said conductive carbon layer including

(A) carbon black particles;

(B) graphite powder;

(C) expanded graphite particles, and

(D) titanium dioxide particles, said titanium dioxide particles of said conductive carbon layer being chemically bonded to said expanded graphite particles.

2. The photovoltaic cell of claim 1, wherein said conductive carbon layer is a sintered conductive carbon layer.

3. The photovoltaic cell of claim 1, wherein said porous film is sintered to said conductive coating.

4. The photovoltaic cell of claim 1, wherein said conductive carbon layer is in a state of rigid attachment to said porous film.

5. The photovoltaic cell of claim 1, the cell further comprising:

(f) a porous separator layer interdisposed between said porous film of said anode and said catalytic component of said cathode, said porous separator layer adapted to physically separate and electrically insulate between said porous film and said cathode.

6. The photovoltaic cell of claim 5, said porous separator layer including a spacing element that is physically distinct with respect to said porous film of said anode.

7. The photovoltaic cell of claim 5, said porous separator layer including a spacing element that is bonded to said porous film of said anode.

8. The photovoltaic cell of claim 5, said porous separator layer including a spacing element that is sintered to said porous film of said anode.

9. The photovoltaic cell of claim 6, said spacing element selected from the group of structural elements consisting of glass fiber and microporous polymer.

10. The photovoltaic cell of claim 1, wherein said chemically bonded titanium dioxide nanoparticles include sintered titanium dioxide nanoparticles.

11. The photovoltaic cell of claim 5, wherein said conductive carbon layer is in a state of rigid attachment with respect to said porous separator layer.

12. The photovoltaic cell of claim 11, wherein said conductive carbon layer is chemically bonded to said porous separator layer.

13. The photovoltaic cell of claim 12, wherein said conductive carbon layer is sintered to said porous separator layer.

14. The photovoltaic cell of claim 1, the cell further comprising:

(f) at least one metal strip or wire, electrically associated with said anode and with said conductive coating, said strip or wire having sufficient thickness to form a protrusion protruding above a plane of said porous film by at least 50 micrometers.

15. The photovoltaic cell of claim 14, said at least one metal strip or wire bonded and to said transparent conductive coating by a conductive binding material.

16. The photovoltaic cell of claim 15, said conductive binding material including a ceramic adhesive.

17. The photovoltaic cell of claim 15, said conductive binding material including titanium nitride.

18. The photovoltaic cell of claim 14, said at least one metal strip or wire is a plurality of wires, and said protrusion is a plurality of protrusions.

19. The photovoltaic cell of claim 1, said conductive carbon layer further including:

(E) zirconium oxide particles, distributed among said carbon black particles, said graphite powder, said expanded graphite particles, and said titanium dioxide particles.

20. The photovoltaic cell of claim 1, said carbon black particles and said expanded graphite particles having a weight ratio in a range of 1.5: 1 to 1 : 1.5.

21. The photovoltaic cell of claim 1, wherein a total carbon content of said conductive carbon layer includes said graphite powder, said carbon black particles, and said expanded graphite particles, said total carbon content and said titanium dioxide particles having a weight ratio in a range of 1.2: 1 to 3.5: 1.

22. The photovoltaic cell of claim 19, wherein a total carbon content of said conductive carbon layer includes said graphite powder, said carbon black particles, and said expanded graphite particles, a weight ratio of said total carbon content and said zirconium oxide particles to said titanium dioxide particles falling in a range of 1.5:1 to 4:1.

23. The photovoltaic cell of claim 1, wherein said graphite powder have a weight ratio with respect to said carbon black particles and said expanded graphite particles, said weight ratio falling in a range of 1.5: 1 to 3.5: 1.

24. The photovoltaic cell of claim 19, wherein said graphite powder has a weight ratio with respect to a sum of said carbon black particles, said expanded graphite particles, and said zirconium oxide particles, said weight ratio falling in a range of 1.2:1 to 3:1.

25. The photovoltaic cell of claim 19, wherein a total carbon content of said conductive carbon layer includes said graphite powder, said carbon black particles, and said expanded graphite particles, a weight ratio of said total carbon content and said zirconium oxide particles to said titanium dioxide particles falling in a range of 1.5:1 to 4:l.

26. The photovoltaic cell of claim 1, wherein a width of said conductive carbon layer is at least 5 cm.

27. The photovoltaic cell of claim 1, wherein a width of said conductive carbon layer is at least 8 cm.

28. The photovoltaic cell of claim 1, said expanded graphite particles having a long dimension of at least 25 micrometers, on average.

29. The photovoltaic cell of claim 1 , said expanded graphite particles having a long dimension of at least 40 micrometers, on average.

30. The photovoltaic cell of claim 1, said titanium dioxide particles having an average primary particle size below 50 nm.

31. A method of producing a photovoltaic cell for converting a light source into electricity, the method comprising the steps of:

(a) providing:

(i) a housing adapted to enclose the photovoltaic cell, said housing including an at least partially transparent cell wall, said cell wall having an interior surface;

(ii) an at least partially transparent conductive coating disposed on said interior surface of said cell wall, within the photovoltaic cell;

(iii) an anode disposed on said conductive coating, said anode including a porous film adapted to make intimate contact with a redox species, said porous film including titanium dioxide nanoparticles, and

(b) providing a cathode generally opposite said anode, said cathode including a sintered conductive carbon layer, said conductive carbon layer including:

(i) carbon black particles;

(ii) graphite powder;

(iii) expanded graphite particles, and

(iv) titanium dioxide particles, said carbon black particles, said graphite powder, said expanded graphite particles, and said titanium dioxide particles being intimately dispersed, said titanium dioxide particles of said conductive carbon layer being chemically bonded to said expanded graphite particles.

32. The method of claim 31, wherein, prior to step (b), the method further comprises the step of:

(c) disposing a conductive carbon paste opposite said anode, said paste including:

(i) carbon black particles;

(ii) graphite powder;

(iii) expanded graphite particles,

(iv) at least one ceramic oxide precursor, said precursor selected from the group of precursors consisting of a titania precursor and a zirconia precursor, and

(v) a liquid vehicle, wherein said sintered conductive carbon layer is produced by sintering said conductive carbon paste at a temperature above 300C, to convert said at least one ceramic oxide precursor to sintered ceramic oxide particles.

33. The method of claim 32, wherein said disposing said conductive carbon paste opposite said anode is performed by screenprinting said paste.

34. The method of claim 31, further comprising the step of:

(c) disposing a porous, electrically insulative separator on said anode, to electrically insulate between said anode and said cathode.

35. The method of claim 32, further comprising the step of:

(d) disposing a porous, electrically insulative separator on said anode, to electrically insulate between said anode and said cathode, wherein said disposing of said conductive carbon paste is performed directly on a surface of said porous, electrically insulative separator.

36. The method of claim 32, wherein said ceramic oxide precursor includes a titanium or zirconium chelate.

37. The method of claim 32, wherein said ceramic oxide precursor is a titania precursor.

38. The method of claim 37, wherein said titania precursor includes an organic titanate.

39. The method of claim 37, wherein said titania precursor includes a compound selected from the group of titania precursors consisting of organic titanium chelates, aqueous titanium chelates, organic titanates and ortho-titanate esters.

40. The method of claim 37, wherein said titania precursor includes a titanium acetylacetonate.

41. The method of claim 32, wherein said liquid vehicle includes an alcohol.

42. The method of claim 32, wherein said sintering of said conductive carbon paste to convert said ceramic oxide precursor to sintered ceramic oxide articles is performed in an air-containing environment, said ceramic oxide precursor includes a fumeless precursor that reacts in said air-containing environment in a substantially non-fuming fashion.

43. The method of claim 32, wherein said cathode includes a support matrix structurally supporting said conductive carbon layer and sintered thereto.

44. The method of claim 31, said conductive carbon layer including:

(v) zirconia particles, intimately dispersed within said conductive carbon layer.

45. The method of claim 32, said conductive carbon layer including:

(vi) zirconia particles, intimately dispersed within said conductive carbon paste.

46. The method of claim 32, further comprising the step of:

(d) disposed within said cell wall, an electrolyte, said electrolyte containing an iodine-based redox species.

47. The method of claim 32, wherein said screenprinting of said paste is performed over a cell width of at least 5 cm.

48. The method of claim 31, said sintered conductive carbon layer being produced by sintering said conductive carbon paste at a temperature above 350C.

49. The method of claim 31, said sintered conductive carbon layer being produced by sintering said conductive carbon paste at a temperature above 400C.

50. A method of producing a photovoltaic cell for converting a light source into electricity, the method comprising the steps of:

(a) providing:

(i) a housing adapted to enclose the photovoltaic cell, said housing including an at least partially transparent cell wall, said cell wall having an interior surface; (ii) an at least partially transparent conductive coating disposed on said interior surface of said cell wall, within the photovoltaic cell; (iii) an anode disposed on said conductive coating, said anode including a porous film adapted to make intimate contact with a redox species, said porous film including titanium dioxide nanoparticles,

(b) disposing a conductive carbon paste generally opposite said anode, said paste including:

(A) carbon black particles;

(B) graphite powder;

(C) expanded graphite particles,

(D) at least one ceramic oxide precursor, said precursor selected from the group of precursors consisting of a titania precursor and a zirconia precursor, and

(E) a liquid vehicle, said carbon black particles, said graphite powder, said expanded graphite particles, and said titanium dioxide particles being intimately mixed, and

(c) sintering said conductive carbon paste at a temperature above 300C, to convert said at least one ceramic oxide precursor to ceramic oxide particles, and producing a conductive carbon cathodic layer from said conductive carbon paste.

51. The method of claim 50, wherein said disposing said conductive carbon paste opposite said anode is performed by screenprinting said paste.

52. A method of producing a photovoltaic cell for converting a light source into electricity, the method comprising the steps of:

(a) providing:

(b) providing a conductive carbon paste, said paste including:

(A) carbon black particles;

(B) graphite powder;

(C) expanded graphite particles;

(E) a liquid vehicle, said carbon black particles, said graphite powder, said expanded graphite particles, and said titanium dioxide particles being intimately mixed;

(c) disposing said conductive carbon paste on a matrix, and

(d) sintering said conductive carbon paste at a temperature above 300C, to convert said at least one ceramic oxide precursor to ceramic oxide particles, and to produce a cathodic layer in which said ceramic oxide particles are sintered to said expanded graphite particles, and in which said ceramic oxide particles are sintered to said matrix.

53. The method of claim 52, wherein said matrix is selected from a group of matrices consisting of glass fiber mat, carbon fiber mat, zirconia felt, titania felt and carbon foam.