WO2009027977A2 - Photovoltaic dye cell having an improved counter-electrode - Google Patents

Abstract

A photovoltaic dye cell for converting a light Into electricity includes an at least partially transparent housing, an electrolyte containing an iodine based redox species, a porous titania film that makes contact with the redox species, and a dye absorbed on a surface of the titania film to convert photons to electrons. The cell also includes at least one flexible conductive carbon sheet to transfer electrons to a current collector associated with the cathode, where the conductive carbon sheet forms a discrete layer conforming to the contours of the titania film.

Description

Photovoltaic Dye Cell Having an Improved Counter-Electrode

This application draws priority from U.S. Provisional Patent Application Serial No. 60/963,316 filed August 28, 2007, from PCT Application No. IL2008/000856, filed June 24, 2008, 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, also known as dye- sensitized solar cells, for producing electricity from sunlight.

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 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 separate, parallel, narrow elongated strips on a common transparent substrate. Each element includes a light facing anode comprising nanocrystalline titania, a carbon counter-electrode (cathode), which is a porous, catalytic, electrically conducting carbon-based structure bonded together using a titania binder, and separating the anode from the cathode is placed an intermediate electrically insulating porous layer based on alumina, silica titania or zirconia powder. 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 clement is connected with the intermediate conducting layer of the preceding anode clement, over a gap separating the respective intermediate layers of these two elements. The series of cells is then sealed using an organic polymer, ensuring in particular that each individual strip cell is sealed from its neighbor cell, and this assembly is referred to as a monolithic assembly of cells.

Generally, dye cells of the above-referenced patents are much closer conceptually to battery cells than to conventional photovoltaic cells, since the charge generators are separated by an electrolyte and are not in direct contact. These 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 10-50 run, applied by baking onto the conductive glass or 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 at least 450 C, 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 efficiency. It is important to note that the titania is principally in contact with the tin oxide. Presence of other conductors (such as many metals, carbon and the like, even if chemically inert to the electrolyte) on the photoanode 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 in that they exhibit chemical inertness to the electrolyte and are substantially free from recombination effects.

For cells that are partially transparent, the opposing electrode ("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, the counter-electrode can be 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 classical cells being dissolved iodine and potassium iodide-essentially potassium tri-iodide. Other solvents, salts and phases, for example, ionic liquids having substantially 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 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 tri-iodide to iodide in the electrolyte, and the iodide is oxidized by the activated dye at the photoanode back to tri-iodide, 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%, and over 11% has been achieved in small champion research cells. 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 70OmV and a current density of 15mA/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, because a high light transmittance through to the dye/titania layer of the photoanode must be maintained. Moreover, tin oxide is only semiconductive, having a resistivity of about 5 x 10⁴ ohm cm, and in addition, is difficult to adhesively bond to. Consequently, in such a cell design, the current takeoff is significantly limited to very small sized cells or to strip cells having long narrow strips of active titania.

It should be noted that active strip cells have certain technical disadvantages. For example, in the cells of Kay described above, the strips of titania and the corresponding strips of carbon are disadvantageously narrow (typically only 6-8mm wide), due to the ohmic loss restriction. This results in a current-limited cell construction as well as an excessive loss of active area between cells, the latter loss due to the practical width of inert materials needed for inter-cell sealing. In any event, adequate sealing between adjacent cells so as to effectively prevent any inter-cell 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 strips disposed on spaced, glass support panes is disclosed by U.S. Patent No. 6,462,266 to Kurth. The conducting strips, which are based on silver or a silver alloy, or on copper or a copper alloy, are printed in paste form and subsequently fired at elevated temperatures. These conducting strips are supposed to be protected from attack by the cell electrolyte by means of an insulating coating consisting of a glass free of heavy metals. The photovoltaic cell taught by U.S. Patent No. 6,462,266 has reduced ohmic loss with respect to the cell disclosed by U.S. Patent No. 5,350,644 to Graetzel, et al., because the conductor strips are good conductors (e.g., silver paste screen-printed on and fired at 600°C), 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 of the tin oxide coating and the long-term strength properties of the glass. Glaze materials that can be processed at lower temperatures are available, but contain toxic heavy metals such as lead, and may also be attacked by the electrolyte or may contaminate the electrolyte. 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 rapidly 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. U.S. Patent Application Publication No. 20050072458 to Goldstein 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 (substantially 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, such that after curing, good electrical contact is made with the tin oxide layer. The wires protrude from the cell from the groove extremities at the glass edges and may be welded to a current-collecting strip.

In a separate embodiment there is described a set of parallel strips of a metal, or a metal alloy that is plated onto the conductive glass or conducting plastic surface. The plated metal resists corrosion under the extremely corrosive operating conditions of the cell. One example given of a plated metal is chromium. Current take-off from the anode plate is again made from the side of the cell, where the plated strips pass sealably through the edge seal of the cell.

In a still further embodiment of the published application, the parallel conductors are inert strips or wires of titanium, molybdenum, tungsten, chromium, or alloys thereof. These robust conductors are 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 of the cell. The conductive 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. Broad dye cells of at least 10- 15cm per side are made possible. Such a cathode, however, although fitted with adequate conducting means for current takeoff from a large area broad cell, obliges a second layer of conducting glass in the cell, with associated cost, weight, and thickness penalties.

The presence of two physically-separated, current-carrying electrodes in the cell may cause additional problems. It is difficult to ensure close and uniform spacing (and hence uniform current distribution) between electrodes in large area cells with such spaced electrodes. Also, due to thermal cycling between full sunlight and night (or freezing) conditions, there may occur aberrant variations in interelectrode spacing and stressing of cell seals, especially for cell electrolytes having an appreciable vapor pressure.

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, long term stability of 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 oxide, rather than using commercially available conductive glass in bulk, such as fluorine doped tin oxide (FTO) glass.

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.

Another problem in prior art cells, especially if the counter-electrode is a separate element from the anode, is the relatively large spacing between anode and cathode - in some cases approaching 100 micrometers for broad cells of industrial applicability- which results in excessive ohmic loss from the electrolyte. It should be evident from the above that when the counter-electrode is a separate element from the anode in the cell, the counter-electrode usually requires the use of a second glass support in the cell, or even more disadvantageously, a second conductive glass in the cell. These greatly add to the cost, weight and thickness of the cell. SUMMARY OF THE INVENTION

According to the teachings of the present invention there is provided a photovoltaic dye cell for converting a light source into electricity at a reduced ohmic loss, including: (a) a housing adapted to enclose the photovoltaic cell, including an at least partially transparent cell wall; (b) an electrolyte, disposed within the cell wall, containing an iodine based redox species; (c) an at least partially transparent conductive coating disposed on an interior surface of the wall; (d) an anode disposed on the conductive coating, including: (i) a porous titania film adapted to make intimate contact with the redox species, and (ii) a dye, absorbed on a surface of the porous titania film, the dye and film adapted to convert photons to electrons; (e) a cathode disposed substantially opposite the anode, having a conductive carbon layer adapted to transfer electrons to a current collection component associated with the cathode, the conductive carbon layer disposed in electrolytic communication, via the electrolyte, with the titania film, the cathode having a surface disposed in an immediately adjacent manner with respect to the surface of the titania film, and wherein an average distance between a surface of the cathode and the surface of the porous titania film is less than 7 micrometers.

According to another aspect of the present invention there is provided a photovoltaic dye cell for converting a light source into electricity at a reduced ohmic loss, the cell including: (a) a housing adapted to enclose the photovoltaic cell, the housing including an at least partially transparent 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 titania film adapted to make intimate contact with the redox species, and (ii) a dye, absorbed on a surface of the porous titania film, the dye and the film adapted to convert photons to electrons; (e) a cathode disposed substantially opposite the anode, the cathode including at least one flexible conductive carbon sheet adapted to transfer electrons to a current collection component associated with the cathode, the conductive carbon sheet disposed in electrolytic communication, via the electrolyte, with the porous titania film, the conductive carbon sheet forming a discrete layer with respect to the porous titania film of the anode, and wherein the flexible conductive carbon sheet adapts to a contour of the porous titania film. According to yet another aspect of the present invention there is provided a photovoltaic dye cell for converting a light source into electricity at a reduced ohmic loss, the cell including: (a) a housing adapted to enclose the photovoltaic cell, the housing including an at least partially transparent cell wall; (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 an interior surface of the cell wall, within the photovoltaic cell; (d) an anode disposed on the conductive coating, the anode including: (i) a porous titania film adapted to make intimate contact with the redox species, and (ii) a dye, absorbed on a surface of the porous titania film, the dye and film adapted to convert photons to electrons; (e) a current collection component, disposed within the housing; (f) a cathode disposed substantially opposite the anode, the cathode including at least one conductive carbon sheet (or carbon layer) adapted to transfer electrons to the current collection component, the conductive carbon sheet disposed in electrolytic communication, via the electrolyte, with the porous titania film, and wherein the current collection component includes a graphite foil or sheet or another chemically inert, electrically conducting foil or sheet made of a conducting material having a specific resistivity below 1200 microohm cm, disposed generally in parallel with the conductive carbon sheet, the graphite foil adapted to collect current from the cathode.

According to further features in the described preferred embodiments, the cathode further includes a catalytic component, associated with the conductive carbon layer and adapted to catalyze a redox reaction of the redox species.

According to still further features in the described preferred embodiments, the distance is achieved over at least 80%, at least 85%, at least 90%, or at least 95% of the surface area of the cell. According to still further features in the described preferred embodiments, the average distance between the surface of the cathode and the surface of the porous titania film is less than 5 micrometers, or even less than 3 micrometers.

According to still further features in the described preferred embodiments, the cathode directly contacts the surface of the porous titania film. According to still further features in the described preferred embodiments, the photovoltaic 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 form a protrusion protruding above a plane of the porous titania film by at least 50 micrometers.

According to still further features in the described preferred embodiments, the conductive carbon layer forms at least one self-supporting sheet.

According to still further features in the described preferred embodiments, the self-supporting sheet is solely physically associated with the porous titanium film of the anode.

According to still further features in the described preferred embodiments, a footprint of a single cell of the photovoltaic cell is defined by a length and a width of the single cell, and wherein both the length and the width exceed 5 centimeters, and in some cases, at least 8 centimeters, or even at least 10 centimeters.

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 forms a plurality of self-supporting strips, the strips disposed between the protrusions.

According to still further features in the described preferred embodiments, wherein the strips are disposed between the protrusions to make a surface of the conductive carbon layer substantially flush with the surface of the porous titania film. According to still further features in the described preferred embodiments, the conductive carbon layer is disposed within a porous support matrix.

According to still further features in the described preferred embodiments, the conductive carbon layer is supported by a flexible porous support matrix, disposed within the conductive carbon layer. According to still further features in the described preferred embodiments, the conductive carbon layer and the flexible porous support matrix conform to contours of the porous titania film.

According to still further features in the described preferred embodiments, the surface of the cathode includes a catalytic component adapted to catalyze a redox reaction of the redox species.

According to still further features in the described preferred embodiments, the flexible porous support matrix includes a fiber mat.

According to still further features in the described preferred embodiments, the flexible porous support matrix includes glass fibers.

According to still further features in the described preferred embodiments, the flexible porous support matrix includes a glass fiber mat.

According to still further features in the described preferred embodiments, the conductive carbon layer includes glass fiber strips impregnated with carbon.

According to still further features in the described preferred embodiments, the glass fiber strips impregnated with carbon directly contact the surface of the porous titania film.

According to still further features in the described preferred embodiments, the partially transparent conductive coating is tin oxide.

According to still further features in the described preferred embodiments, the photovoltaic cell further includes at least one electrically-conductive structural component, associated with the conductive carbon layer, the structural component having a specific resistivity below 1200 microohm-cm, the component adapted and disposed to collect current from the cathode.

According to still further features in the described preferred embodiments, the electrically-conductive structural component includes graphite.

According to still further features in the described preferred embodiments, the electrically-conductive structural component includes a graphite foil. According to still further features in the described preferred embodiments, the photovoltaic cell further includes a metal conducting element, attached to the graphite foil, the metal conducting element extending through a side wall of the cell to effect current take-off.

According to still further features in the described preferred embodiments, the metal conducting element is a metal foil or a metal mesh.

According to still further features in the described preferred embodiments, less than 50%, less than 30%, less than 20%, or even less than 10% of the electrolyte is disposed between the anode and the cathode.

According to still further features in the described preferred embodiments, the flexible conductive carbon sheet has a Shore D hardness below 90, below 80, or even below 70.

According to still further features in the described preferred embodiments, the current collection component contains less than 2% binder, by weight. Preferably, the current collection component contains less than 1% binder, by weight, and more preferably, less than 0.5% binder, by weight. The current collection component may be substantially binderless.

According to still further features in the described preferred embodiments, the photovoltaic cell further includes a metal conducting element, attached to the graphite foil, the metal conducting element extending through a side wall of the cell to effect current take-off.

According to still further features in the described preferred embodiments, the metal conducting element is a metal foil or a metal mesh. The photovoltaic dye cells of the present invention may be simple, broad, large- area, efficient, low-cost, lightweight and robust, and may successfully address the various shortcomings of the prior art.

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:

Figure 1 is a schematic cross-sectional view of an "open-faced" photovoltaic dye cell, according to one preferred embodiment of the present invention;

Figure 2 provides a schematic cross-sectional view of a dye cell according to another preferred embodiment of the invention;

Figure 3 a provides a schematic cross-sectional view of an inventive photovoltaic dye cell having a bi-plate structure;

Figure 3b is a schematic top view of the cell of Figure 3a, showing the disposition of the anchoring points of the cell;

Figure 4 is a schematic cross-sectional view of a photovoltaic dye cell having a bi- plate structure, according to another preferred embodiment of the present invention, and

Figure 5 provides a schematic cross-sectional view of a photovoltaic dye cell having conductor- filled grooves as cathodic current take-off means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principles and operation of the counter-electrode and the photovoltaic dye cell of 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. One aspect of the present invention is a counter-electrode for broad, large-area, single dye cells of typically 15 cm per side, and an inventive photovoltaic cell structure incorporating such a counter-electrode. We have found that when broad, single cell assemblies of this type are attempted with the usual carbon-based compositions of the prior art counter-electrode (as exemplified by the above-referenced patent to Kay, which uses titania powder as binder to bond the carbon layer), there emerges, in addition to the expected current takeoff limitation with cell widths much above 1 cm, an additional problem resulting from the rather fragile nature of the carbon layer. It is very difficult to embed, even partially, a current-collecting structure such as a metal mesh, a metal foil or metal wires in a broad area carbon layer, without causing the entire structure to delaminate. This is principally due to the non-elastic nature of inorganic binders such as titania used to bond the carbon layer. More flexible organic type binders are generally unsuitable for this application because of the high sintering temperature of 450 C needed for both anode and cathode layers in the cell. The high sintering temperature may destroy organic binders and possibly contaminate the cell with organic decomposition residues. Furthermore, many metals are corroded by the cell electrolyte under the dye cell working conditions and thus appear unsuitable for use in the counter-electrode as current collectors. In the dye-cell technology taught by Kay, these issues do not arise because of the narrow width of the strip cells. In such narrow cells:

• the sintered-on carbon layer may display adequate mechanical stability, and • the carbon layer alone may convey current, without the need for a supplemental current collector.

The cell structure of the present invention may include a graphite foil, in combination with a corrosion-resistant metal or metal alloy current takeoff element at least partly embedded in the graphite foil, which structure is conductively bonded to, or in direct electrical conductive contact with, an underlying cathode carbon layer. This structure is acceptably robust and enables a large area, broad cell construction, while simultaneously eliminating the need for a second layer of conducting glass in the dye cell or a second, physically separate counter-electrode. The corrosion-resistant current takeoff element embedded in the graphite foil may be selected from various geometries including a foil, mesh, strips or wires of a number of metals or alloys such as titanium, titanium-clad copper, tungsten, a higher alloy of iron and chromium, or a higher alloy of iron, chromium and molybdenum.

In another preferred embodiment, the corrosion-resistant metal or metal alloy current takeoff element may be used as a standalone counter-electrode in the cell, without the need for combination with a carbon or graphite foil layer, and in such a case, the counter-electrode catalyzed directly with trace platinum.

A schematic cross-sectional view of one embodiment of an inventive photovoltaic dye cell 100, which may be of an "open-faced" sandwich design, is shown in Figure 1. Onto a support glass such as a conventional anode support glass 102 (of typical thickness lmm-3mm), a thin, transparent conducting surface layer 104 is disposed, the layer based, for example, on tin oxide. Support glass 102 and conducting surface layer 104 may be provided with electrical conductivity enhancing features (e.g., as described in above- referenced U.S. Patent Application Publication No. 20050072458, which is incorporated by reference for all purposes as if fully disclosed herein). Support glass 102 is used as the substrate and basic building block for the dye cell. The substrate is grooved and fitted with wire current take-off means as taught in the above-referenced patent publication. Thus, wires pass sealably out of the cell through the side of the cell, and are braided together and/or connected, for example, by welding, to a current collecting strip (not shown), so as to form the cell anode terminal. In each groove 108 there is applied an inert, electrically conducting binder composition 112 that bonds a wire 116 into the groove and makes good bridging electrical contact with the tin oxide layer on each side of the groove. As an example, a titanium, tungsten, or higher alloy wire may be inserted into the groove. The conducting binder, which preferably includes a conductor such as titanium nitride and a binder such as alumina, is also added into the groove. Onto the tin oxide surface is applied a nanocrystalline titania layer, typically by screen-printing from a paste, followed by drying and sintering. Porous, sintered nanocrystalline titania film or layer 120 is designed to have a typical thickness of about 15 micrometers. Titania film or layer 120 may include several sublayers of titania, each of which may be individually screen-printed and sintered. We have found that covering the upper surface of the conducting binder 112 with an insulating layer 124 prior to the application of the nanocrystalline titania layer, may help to eliminate possible short circuits developing between the anode and the cathode.

Insulating layer 124 may be especially important for cells in which the conducting binder surface is appreciably above the level of sintered titania layer 120, as depicted in Figure 1.

Insulating layer 124 may be selected from a glaze or a binder composition containing fairly coarse titania, zirconia, alumina or silica particles preferably having a characteristic particle size on the order of several micrometers, or alternatively, from a high temperature polymer such as polyimide or silicone. Titania layer 120 may then be covered with a screen-printed, porous, insulating spacer layer from a paste containing relatively coarse titania, zirconia silica or alumina particles. The screen-printed spacer layer is designed to yield a sintered insulating spacer layer 128 having a thickness of about 5 micrometers. Sintered insulating spacer layer 128 also acts as a light scattering layer, directing light back to sintered nanocrystalline titania layer 120.

Cell construction continues with application of the counter-electrode layers, initially by screen-printing. Spacer layer 128 may be narrow, typically on the order of 2 - 10 micrometers, to ensure a very small anode/cathode separation in the cell, a low ohmic resistance, and high cell fill factor, all of which contribute to increased cell performance. Onto insulating spacer layer 128 may be applied a thin porous layer of carbon, optionally catalyzed with a trace amount of platinum or a platinum catalyst substitute via a screen- printable paste containing an inert binder. This thin layer is dried and sintered to produce a sintered, porous, catalytic carbon layer 132, which has a characteristic thickness of only a few micrometers.

The sintering of the screen-printable, porous, catalytic carbon paste layer is followed by the application of a relatively thick layer of porous, conducting carbon via a screen-printable carbon paste containing an inert binder. Another sintering step is performed, producing a sintered, porous, conducting carbon layer 136. The screen- printing of the carbon paste is typically designed such that the thickness of sintered, conducting carbon layer 136 is typically about 50-100 micrometers.

Conducting carbon layer 136 may be sufficiently active for the iodine redox reaction in the cell such that the need for a separate catalytic layer such as catalytic carbon layer 132 may be obviated.

In a preferred embodiment, the various layers may be sintered, if possible in a single sintering step at 450°C, and following partial cooling, a sensitizer dye is introduced into titania layer 120 via porous carbon layer 136. A sheet of graphite foil 140 having a prepared, sintered-on, porous, conducting carbon (and chemically inert) binder layer 144 disposed thereunder may be laid onto carbon layer 136 so as to be in good conductive contact therewith.

Porous carbon layer 136, graphite foil 140 (including binder layer 144), and optionally, catalytic layer 132 may form a cathode or counter-electrode 160 of cell 100. At least partially embedded into the graphite foil may be a corrosion-resistant metal or metal alloy current collector 152 in the form of a wire, mesh, strip, perforated strip or foil that protrudes through the peripheral cell seal (described hereinbelow) to act as a counter-electrode terminal of the cell. We have found that under normal cell operating conditions, titanium, titanium-clad copper, and tungsten, and some higher alloys principally consisting of chromium and iron, or chromium, iron and molybdenum, may serve as chemically stable counter-electrode current collecting materials.

Graphite foil 140 may advantageously be equipped with one or more perforations 146 to facilitate electrolyte distribution into the cell during a subsequent filling operation. A sheet 154 may be laid on top of the assembly to close the cell. Sheet 154 may be made of inexpensive window glass or various metals or alloys. Underneath sheet 154, and on top of graphite foil 140, may advantageously be disposed an elastic sheet 148, preferably including or consisting of foam, fiber mat or elastomer mat or a swelling precursor material or swelling polymeric material. Elastic sheet 148, which may include a polymeric, carbon or metallic material, is compressible and spring-like, to help maintain a fairly uniform pressure between graphite foil 140 and carbon layer 136 while ensuring sufficient electrical contact over the large requisite area, without delaminating layer 136.

The edges of cell 100 may be sealed by a peripheral seal 156, e.g., using a liquid- phase sealant. Electrolyte may then be introduced into cell 100, typically by vacuum means via a hole (not shown) in sheet 154, and the hole may be sealed using a sealing composition. The cell is then ready for testing/modulizing.

A schematic cross-sectional view of a photovoltaic dye cell 200 according to another preferred embodiment of the invention, is shown in Figure 2. On a support glass such as a conventional anode support glass 202 (of typical thickness lmm-3mm), a thin, transparent conducting surface layer 204 is disposed, the layer based, for example, on tin oxide. Support glass 202, which may be provided with electrical conductivity enhancing features, serves as the substrate and basic building block of dye cell 200. Disposed on conducting surface layer 204 is a set of spaced, preferably substantially parallel strips 216 of a metal or alloy inert to the cell electrolyte and to charge carrier recombination. Strips 216, which may be deposited by means of electroplating, provide current take-off means for cell 200. Strips 216 may pass sealably through the side of cell 200, and may be electrically connected together, e.g., by a current collecting strip (not shown) outside of the seal, thereby forming the anode terminal of the cell. Substantially as described hereinabove with respect to the filled grooves provided in Figure 1, the plated strips may be also advantageously coated with an insulating layer 228, for example, including a glaze or a binder composition of titania, zirconia, alumina and/or silica, to prevent anode/cathode short-circuiting. Onto conducting surface layer 204 is applied a nanocrystalline titania layer, typically by screen-printing from a paste, drying and sintering.

Porous, sintered nanocrystalline titania film or layer 220 is designed to have a typical thickness of about 15 micrometers. Titania film or layer 220 may include several sublayers of titania, each of which may be individually screen-printed and sintered.

Titania layer 220 is then covered with a screen-printed, porous, insulating spacer layer, which undergoes sintering to produce a sintered insulating spacer layer 228. The screen-printed paste may contain relatively coarse titania, zirconia, silica, and/or alumina particles. After drying and sintering, sintered insulating spacer layer 228 may have a thickness of about 2 - 10 micrometers. This thickness is designed to ensure a very small anode/cathode separation in the cell. This thickness is further designed to achieve a low internal resistance and a high cell fill factor, thereby raising cell performance. Spacer layer 228 also acts as a light back-scattering layer, directing light back to sintered titania layer 220.

On insulating spacer layer 228 is disposed a cathode or counter-electrode 260 of cell 200. Cathode 260 includes a sintered, porous, catalytic conducting carbon layer 232, which may contain a binder that is chemically inert to the constituents of cell 200, and which may be catalyzed with a trace amount of platinum or of a platinum catalyst substitute. Carbon layer 232 may be produced by screen-printing or otherwise applying a thin layer of porous, catalytic carbon paste, after which the layer is dried and sintered. Sintered, porous, conducting carbon layer 232 may have a thickness of several micrometers, typically less than about 10 micrometers. Disposed above catalytic carbon layer 232 is a relatively thick, sintered, conducting carbon layer 236, which may also form a part of cathode 260. Carbon layer 236 may be produced by screen-printing, after which the layer is dried and sintered to achieve a typical thickness of 50-100 micrometers.

Some high surface area carbons and graphites are so active for the iodine redox reaction in the cell that the need for a separate catalytic or platinized layer 228 may be obviated. In a preferred embodiment, the various layers are sintered, typically or preferably in a single sintering step at 450 C, and following partial cooling, a sensitizer dye is introduced into titania layer 220 via porous carbon layer 236. A sheet of graphite foil 240 is then conductively bonded to carbon layer 236 via a conducting carbon adhesive layer 245, which may be selected to be curable at a temperature below 120 C, thereby avoiding any damage to heat-sensitive dyes in titania layer 220. Suitable components for adhesive layer 245 may include carbon powder and either an inert (with respect to the electrolyte) inorganic binder based on alumina, or an inert (with respect to the electrolyte) organic binder based on silicone or polyimide, for example.

Both graphite foil 240 and conducting carbon adhesive layer 245 may be considered to form a part of cathode 260.

Into graphite foil 240 is embedded, at least partially, a corrosion-resistant metal or metal alloy current collector 252 in the form of a wire, mesh, strip, perforated strip or foil that passes out through the peripheral cell seal (described below) to act as the counter- electrode terminal of cell 200. We have found that titanium, titanium-clad copper, and tungsten, or some higher alloys principally consisting of chromium and iron, or chromium, iron and molybdenum, may serve as chemically stable counter-electrode current collecting materials. Graphite foil 240 may advantageously have perforations 246 in order to facilitate electrolyte distribution into the cell. The electrolyte-filling step may be carried out at this stage.

The cell is structurally completed by laying down a sheet of plastic laminated foil 258 and applying a peripheral sealant 252 between elements 202 and 258 of the cell, or alternatively a polymer sealing layer may be sprayed on to initially seal the cell, and additional sealing provided using an outer metal foil (not shown). However, if a lightweight construction is not particularly crucial, window glass or metal/alloy sheet can be used to close the cell, and in the latter case, current collector 252 may be dispensed with. As developed hereinabove, large area, broad dye cells having physically separated anode and counter-electrodes may suffer from variously characteristic performance limitations. Reducing or eliminating this physical separation is one motivation behind the open-faced sandwich design provided in Figures 1 and 2, in which all the active layers are bonded onto a single (conductive) glass sheet. In broad cells of industrial applicability, and particularly in cells using non- volatile electrolytes, inter-electrode spacing tends to be at least 50-100 micrometers, at which point ohmic losses from the cell electrolyte become excessive. Similarly, temperature changes between day and night may cause expansion/contraction variations in interelectrode spacing, which may lead to current irregularities and to stressing of seals. In a preferred embodiment, however, we disclose a photovoltaic dye cell having a bi-plate design that may overcome these limitations.

While various sandwich structures, in which a carbon layer is printed on top of a zirconia spacer layer on top of a titania layer, may allow close spacing of carbon and titania surfaces, it must be emphasized that such structures may be extremely prone to penetration of the carbon printing layer to the conducting glass surface, via any holes or defects in the printed layers of titania or spacer. The penetration of the carbon printing layer to the conducting surface of the FTO glass effects short-circuiting of the cell, thereby reducing or limiting cell performance. This problem is particularly acute in view of the at least 7-10 year lifetime often required for cells to attain commercial viability. The inventive biplate construction, as described herein, avoids this significant and potentially critical problem, because the carbon counterelectrode is not sintered (i.e., chemically bonded at high temperature) to the printed layer(s) of titania or spacer. Rather, the carbon counterelectrode is a distinct and separate entity that lies on top of, and may physically contact, the spacer layer, or the upper surface of the titania layer, but is not chemically bonded thereto, as in printed carbon layer technologies.

As used herein in the specification and in the claims section that follows, the term "immediately adjacent", with respect to a surface of the cathode and a surface of the anode (e.g., a carbon or catalyzed carbon surface of the cathode and a porous titania film of the anode) refers to surfaces that are not separated by an interceding layer.

As used herein in the specification and in the claims section that follows, the term "solely physically associated", with respect to layers of the cathode and/or layers of the anode, refers to layers that are in contact, but are not sintered together and are otherwise chemically disattached.

As used herein in the specification and in the claims section that follows, the term "self-supporting", with respect to strips or layers of the cathode and/or layers of the anode, refers to strips or layers that are held in place within the cell in a disattached structure with respect to the opposing electrode. As used herein in the specification and in the claims section that follows, the term

"discrete", with respect to adjacent layers in the cell, refers to layers that may be in contact with one another, yet are physically distinct.

A schematic cross-sectional view of one embodiment of such an inventive photovoltaic dye cell 300 is provided in Figure 3a. An anode glass 302 of typical thickness lmm-3mm has a thin, transparent conducting surface layer 304 based on a conductive material such as tin oxide. Anode glass 302 and conducting surface layer 304 may be provided with electrical conductivity enhancing features. The conductive glass is grooved and fitted with wire current take-off means substantially as described hereinabove. Thus, wires pass sealably out of the cell at the side of the cell and are braided together and/or connected, for example, by welding, to a current collecting strip (not shown), to form an anode terminal of cell 300. In each groove 308 there is applied an electrically-conducting binder layer 312, which is preferably chemically inert to the cell electrolyte and serves to bond a wire 316 into groove 308 while making good bridging electrical contact with the tin oxide layer (conducting surface layer 304) on each side of groove 308. As an example, a titanium, tungsten, or higher alloy wire may be inserted into groove 308 and a conducting binder, which preferably includes a conductor such as titanium nitride and a binder such as alumina, is also added into groove 308. In constructing cell 300, an upper surface of conducting binder layer 312 may be covered with an insulating layer 324 prior to the application of the nanocrystalline titania layer, to prevent short-circuits developing between the anode and the cathode. This is especially important when the surface of conducting binder layer 312 is appreciably above the level of a sintered nanocrystalline titania layer 320, as depicted in Figure 3a. Insulating layer 324 may be selected from a glaze or a binder composition containing fairly coarse titania, zirconia, alumina or silica particles having a characteristic particle size of several micrometers, or alternatively, from a high temperature polymer such as polyimide or silicone. Onto the tin oxide surface (conducting surface layer 304) is applied a nanocrystalline titania layer, typically by screen-printing a paste, followed by drying and sintering to produce sintered nanocrystalline titania layer 320. This sintered layer is designed to have a typical thickness of about 15 micrometers after the sintering step. Titania layer 320 can be optionally coated with a coarse particle layer (not shown) based on titania, zirconia, alumina or silica, for purposes of electrical insulation and/or light back-scattering. During construction of cell 300, titania layer 320 may then be covered with sensitizer dye, or this step can be carried out later on, before electrolyte filling. The cell is closed off with a corrosion-resistant metal sheet 368 that has been catalyzed conventionally with a catalyst such as trace platinum crystallites 332, and this sheet is sealed at the edges to anode glass 300 by electrolyte-resistant sealant 336. Suitable construction materials for metal sheet 368 may include titanium, titanium-clad copper, tungsten, or higher alloys including principally chromium and iron, or chromium, iron and molybdenum.

Importantly, metal sheet 368 is bonded to the upraised, electrically insulated material of insulating layer 324 by means of a layer of electrolyte-resistant adhesive 340. Suitable adhesives are available, based on silicones or polyimides, for example. Adhesive layer 340 may be placed at periodic spacing intervals along the materials covering grooves 308 to maintain a strong anchoring and uniform spacing between sheet 368 and anode glass 302. A lateral continuous layer of adhesive is operational, but may be disadvantageous, however, as it hydraulically isolates electrolyte between adjacent grooves and makes the filling of cell 300 with electrolyte rather laborious.

Figure 3b shows the placing of the anchoring points in a schematic top view of cell 300, in which each "+" mark 303 represents a spaced location along the grooves in the anode plate where adhesive is placed.

Sheet 368 in Figure 3 a may be grooved in complementary fashion to anode glass 302, in order to accept the upraised profile of insulating layer 324 as required and thereby enable the desirable close approach (a few tens of micrometers only) between the surface of catalyst 332 and the surface of titania layer 320. The cell may be filled with electrolyte via holes (not shown) in metal sheet 368, which holes are then sealed off.

Various alternative embodiments in the building of the two-plate design according to the desired spacing criteria are possible. In Figure 3 a, the anode plate may be fitted with conducting metal strips instead of buried wires. Similarly, the metal sheet based counter-electrode may simply be stamped out to give a groove-like topography. In one preferred embodiment, the counter-electrode may be a suitably catalyzed flat metal sheet from which slits have been punched or cut out, such that the slits fit over the upraised elements situated on the grooves, while simultaneously enabling the catalyzed surface between the slits to attain close proximity to the titania surface below. In such a case, a glass or polymer sheet placed above the slitted plate would be used to close off the cell, and this glass or polymer sheet would be anchored in place at multiple points on the upraised elements.

Figure 4 provides a schematic cross-sectional view of a bi-plate photovoltaic dye cell 400 according to another embodiment of the present invention. The construction of this biplate cell enables the use of only a single conducting glass sheet per cell. This construction further enables a very close juxtaposition of anode and cathode. A photoanode support glass or substrate 402 coated with a substantially transparent conductor layer 404 may have spaced, preferably parallel wires 410 bonded in place on a surface of the conductive glass (e.g., on top of conductor layer 404) by means of a substantially inert conducting adhesive layer such as a conducting ceramic adhesive layer 415. Between chemically inert current collecting wires 410 and at the edges of cell 400 is a sintered, nanocrystalline, porous titania layer 406, typically 10-15 micrometers thick. Since wires 410 may typically have a diameter of at least 100 micrometers, and the height of titania layer 406 is typically below about 15 micrometers, wires 410 may project above titania layer 406 by over 80 micrometers. To avoid short-circuiting to a counter-electrode 430 of cell 400, ceramic adhesive layer 415 may advantageously be covered with an electrically insulating layer 418, e.g., a layer including zirconia. Electrically insulating layer 418 may typically have a thickness in a range of 20 -50 micrometers. Cell 400 may have two physically discrete electrodes. Thus, a cathode or counter- electrode 430 may be a distinct component that is laid onto the anode element and may be disposed in close proximity thereto. Counter-electrode 430 may advantageously include a porous, conductive cathodic layer 425, e.g., a porous support matrix supporting impregnated carbon. The porous matrix may include or essentially consist of a mat, woven and/or non- woven, foam, or possibly other matrices known in the art. A preferred material for the mat is glass fiber since it is low cost, flexible and conformable to the cell geometry, chemically inert in the cell environment, and may withstand elevated curing or sintering temperatures. Other fibers such as carbon fibers may also be used. The impregnated carbon may be bonded to the porous support matrix by an inert binder, which may be selected from inorganic materials such as alumina, or polymeric materials such as polytetrafluoroethylene (PTFE, or Teflon^®).

Counter-electrode 430 may include porous cathodic layer 425 and a catalytic layer 432 (catalytic layer 432 being disposed towards titania layer 406), or a combination thereof. Porous cathodic layer 425 may be disposed within cell 400 as a plurality of strips, each strip having a width enabling the strip to fit between neighboring protrusions of layer 418, or, adjacent to the cell perimeter, between a protrusion of layer 418 and an inner wall of an (inner) edge seal 465.

Current takeoff from cathode or counter-electrode 430 may be effected via a graphite foil 435 laid on (and typically physically discrete with respect to) the strips of porous cathodic layer 425. At its periphery, graphite foil 435 may have an at least partially embedded inert metal mesh or foil 440 that may protrude outside cell 400, to facilitate cathodic current collection. Dye cell 400 may be produced according to the following inventive method: onto a surface of photoanode support glass 402 having transparent conductor layer 404 (e.g., an electrically conducting, transparent tin oxide layer), are screen-printed strips of titania paste. Upon sintering, spaced-apart strips of sintered, porous titania layer 406 are produced. In each gap between adjacent titania strips is situated a current collecting wire 410. A chemically inert, electrically conducting ceramic paste may be printed around wire 410. On top of the ceramic material may be printed an electrically insulating paste. A sintering operation causes ceramic adhesive layer 415 to be chemically bonded to transparent conductor layer 404. Sintering s electrically insulating layer 418 to be chemically bonded to ceramic adhesive layer 415.

At this point the titania strips may be stained with dye.

The counter-electrode of cell 400 includes strips of porous, conductive layer 425, which are laid on to the strips of titania layer 406. These strips may advantageously include glass fiber mats impregnated with electrically conducting carbon paste, which have undergone sintering. Strips of conductive layer 425 may be advantageously catalyzed by means of catalytic layer 432, which may include trace platinum catalyst on a surface thereof. These strips may be directly laid on sintered titania layer 406 of the photoanode, with catalytic layer 432 being disposed towards titania layer 406. Despite this direct contact, we have found that no further insulation of titania from the carbon in the carbon-impregnated strips is necessary.

Current takeoff is advantageously accomplished via a graphite foil 435 laid on the strips of conductive layer 425. Graphite foil 435 has an embedded inert metal mesh or foil 440 at a periphery thereof, which can pass sealably out of the cell. Cell 400 is sealed by means of a window glass cover 445 in conjunction with an inner edge seal 465 and an outer edge seal 470, preferably produced by means of two sequentially applied polymers. Glass cover 445 applies adequate pressure to the graphite foil underneath to ensure good electrical contact and close interspacing of the elements in the cell. Electrolyte addition to the cell may be by means of fill holes in the window glass (not shown) which holes are later sealed off by polymer. Cathode current withdrawal from the cell is via a takeoff terminal 442 outside the walls of cell 400, formed by a sealable protrusion of metal mesh or foil 440 through the cell wall, e.g., through edge seals 465 and 470. Individual wires, emerging sealably from the photoanode via edge seals 465 and 470, can be welded to a current collecting strip (not shown) to form the photoanode current collector.

In a solar module, individual cells may be electrically connected and suitably mounted in a support structure.

It is evident from the above that cathode 430 forms a discrete, physically separate layer with respect to porous titania film 406 of the anode. Cathode 430 is also adapted to display a measure of compressibility and elasticity. In sharp contrast to the substantially unyielding cathodes of the prior art, which, to prevent damage, must be held at a distance from the surface of the fragile porous titania film of the anode, cathode 430 (and in particular, strips of conductive layer 425) may be held against the surface of porous titania film 406, and may absorb moderate pressures normal to porous titania film 406, so as to protect titania film 406. Moreover, the bottommost surface of cathode 430 may adapt or conform to a contour of titania film 406, such that the significant ohmic inefficiency characterizing various prior art dye cells is appreciably reduced.

In order to strips of attain the requisite degree of conformability, strips of conductive layer 425 may have a Shore D hardness below 90. In some cases, strips of conductive layer 425 may have a Shore D hardness below 80, or below 70.

In a schematic cross-sectional view of a photovoltaic dye cell 500 according to another preferred embodiment of the invention, shown in Figure 5, the anodic section may be structured substantially as in Figure 3 a. A counter-electrode plate or cover includes a window glass 578, which is bonded at the edges to an anode glass 302 by a peripheral seal 556, e.g., using a liquid sealant. A nanocrystalline titania layer 320 of the anode may be coated with a coarse particle layer 322 based on titania, zirconia, alumina or silica, for purposes of electrical insulation and light back-scattering. Nanocrystalline titania layer 320 may then be covered with a sensitizer dye, or this step can be carried out subsequently, prior to electrolyte filling. On the cathodic side of cell 500, each of at least one groove such as groove 588 contains a wire 596 bonded in place by a conducting adhesive layer 592, which layer extends also as a continuous layer across the glass face in order to provide electrical conductivity between adjacent grooves. The anode and counter plates may be bonded together at various points (e.g., periodically spaced intervals) along anode and counter grooves (as in Figure 3a) by a layer of adhesive 566. Between adjacent grooves, a layer 570 of carbon or graphite, optionally coated with a catalyst such as trace platinum, is provided, and the thickness of this layer is selected so as to allow close proximity of the carbon or catalyst surface to nanocrystalline titania layer 320. Cell 500 may be filled with electrolyte through fill holes (not shown) in the counter-electrode, which is then sealed, at which point cell 500 may be ready for testing and modulizing. The presence of closely placed anchoring points (e.g., about one cm apart on a large area cell sized 15cm x 15cm), and the use of a strong adhesive such as silicone, maintain spacing and prevent short-circuiting within cell 500.

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 dye cell for converting a light source into electricity at a

reduced ohmic loss, the cell comprising:

(a) a housing adapted to enclose the photovoltaic cell, said housing including an at least partially transparent 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 titania film adapted to make intimate contact with said redox species, and (ii) a dye, absorbed on a surface of said porous titania film, said dye and said film adapted to convert photons to electrons;

(e) a cathode disposed substantially opposite said anode, said cathode including a conductive carbon layer adapted to transfer electrons to a current collection component associated with said cathode, said conductive carbon layer disposed in electrolytic communication, via said electrolyte,

with said porous titania film,

said cathode having a surface disposed in an immediately adjacent manner with respect to

said surface of said porous titania film of said anode,

and wherein an average distance between a surface of said cathode and said surface of

said porous titania film is less than 7 micrometers.

2. The photovoltaic cell of claim 1, wherein said cathode further includes a

catalytic component, associated with said conductive carbon layer and adapted to catalyze

a redox reaction of said redox species.

3. The photovoltaic cell of claim 1, wherein said distance is achieved over at

least 80% of the cell surface area.

4. The photovoltaic cell of claim 1 , wherein said distance is achieved over at

least 85% of the cell surface area.

5. The photovoltaic cell of claim 1, wherein said distance is achieved over at

least 90% of the cell surface area.

6. The photovoltaic cell of claim 1 , wherein said distance is achieved over at

least 95% of the cell surface area.

7. The photovoltaic cell of claim 2, wherein said distance is less than 5

micrometers.

8. The photovoltaic cell of claim 2, wherein said distance is less than 3

micrometers.

9. The photovoltaic cell of claim 1, wherein said surface of said cathode

directly contacts said surface of said porous titania film.

10. The photovoltaic cell of claim 1, 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 titania film by at least 50 micrometers.

11. The photovoltaic cell of claim 1 , said conductive carbon layer forming at least one self-supporting sheet.

12. The photovoltaic cell of claim 11, said self-supporting sheet is solely physically associated with said porous titanium film of said anode.

13. The photovoltaic cell of claim 1, wherein a footprint of a single cell of the photovoltaic cell is defined by a length and a width of said single cell, and wherein both said length and said width exceed 5 centimeters.

14. The photovoltaic cell of claim 10, said at least one metal strip or wire is a plurality of wires, and said protrusion is a plurality of protrusions.

15. The photovoltaic cell of claim 14, said conductive carbon layer forming a plurality of self-supporting strips, said strips disposed between said protrusions.

16. The photovoltaic cell of claim 15, said strips disposed between said protrusions to make a surface of said conductive carbon layer substantially flush with said surface of said porous titania film.

17. The photovoltaic cell of claim 1, wherein said conductive carbon layer is disposed within a porous support matrix.

18. The photovoltaic cell of claim 1, wherein said conductive carbon layer is supported by a flexible porous support matrix.

19. The photovoltaic cell of claim 18, wherein said conductive carbon layer and said flexible porous support matrix conform to contours of said porous titania film.

20. The photovoltaic cell of claim 1, wherein said surface of said cathode includes a catalytic component adapted to catalyze a redox reaction of said redox species.

21. The photovoltaic cell of claim 18, wherein said flexible porous support matrix includes a fiber mat.

22. The photovoltaic cell of claim 18, wherein said flexible porous support matrix includes glass fibers.

23. The photovoltaic cell of claim 18, wherein said flexible porous support matrix includes a glass fiber mat.

24. The photovoltaic cell of claim 1, wherein said conductive carbon layer includes glass fiber strips impregnated with carbon.

25. The photovoltaic cell of claim 1, wherein said glass fiber strips impregnated with carbon directly contact said surface of said porous titania film.

26. The photovoltaic cell of claim 1, wherein said partially transparent conductive coating is tin oxide.

27. The photovoltaic cell of claim 1, further comprising at least one electrically-conductive structural component, associated with said conductive carbon layer, said structural component having a specific resistivity below 1200 microohm cm, said component adapted and disposed to collect current from said cathode.

28. The photovoltaic cell of claim 27, wherein said electrically-conductive structural component includes graphite.

29. The photovoltaic cell of claim 27, wherein said electrically-conductive structural component includes a graphite foil.

30. The photovoltaic cell of claim 29, further comprising a metal conducting element, attached to said graphite foil, said metal conducting element extending through a side wall of the cell to effect current take-off.

31. The photovoltaic cell of claim 30, wherein said metal conducting element is a metal foil or a metal mesh.

32. The photovoltaic cell of claim 1, wherein less than 50% of said electrolyte is disposed between said anode and said cathode.

33. The photovoltaic cell of claim 1, wherein less than 30% of said electrolyte

is disposed between said anode and said cathode.

34. The photovoltaic cell of claim 1, wherein less than 20% of said electrolyte

is disposed between said anode and said cathode.

35. The photovoltaic cell of claim 1, wherein less than 10% of said electrolyte

is disposed between said anode and said cathode.

36. A photovoltaic dye cell for converting a light source into electricity at a

reduced ohmic loss, the cell comprising:

(a) a housing adapted to enclose the photovoltaic cell, said housing including an at least partially transparent 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 titania film adapted to make intimate contact with said redox species, and (ii) a dye, absorbed on a surface of said porous titania film, said dye and said film adapted to convert photons to electrons;

(e) a cathode disposed substantially opposite said anode, said cathode including at least one flexible conductive carbon sheet adapted to transfer electrons to a current collection component associated with said cathode, said conductive carbon sheet disposed in electrolytic communication, via said electrolyte,

with said porous titania film,

said conductive carbon sheet forming a discrete layer with respect to said porous titania film of said anode, and wherein said flexible conductive carbon sheet conforms to a contour of said porous titania film.

37. The photovoltaic cell of claim 36, wherein said conductive carbon sheet includes a flexible porous support matrix.

38. The photovoltaic cell of claim 37, wherein said flexible porous support matrix includes a fiber mat.

39. The photovoltaic cell of claim 37, wherein said flexible porous support matrix includes glass fibers.

40. The photovoltaic cell of claim 37, wherein said flexible porous support matrix includes a glass fiber mat.

41. The photovoltaic cell of claim 36, wherein said conductive carbon sheet includes glass fiber strips impregnated with carbon.

42. The photovoltaic cell of claim 41, wherein said glass fiber strips impregnated with carbon directly contact said surface of said porous titania film.

43. The photovoltaic cell of claim 36, wherein said flexible conductive carbon sheet has a Shore D hardness below 90.

44. The photovoltaic cell of claim 36, wherein said flexible conductive carbon

sheet has a Shore D hardness below 80.

45. The photovoltaic cell of claim 36, wherein said flexible conductive carbon

sheet has a Shore D hardness below 70.

46. A photovoltaic dye cell for converting a light source into electricity at a

reduced ohmic loss, 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 titania film adapted to make intimate contact with said redox species, and (ii) a dye, absorbed on a surface of said porous titania film, said dye and said film adapted to convert photons to electrons;

(e) a current collection component, disposed within said housing;

(f) a cathode disposed substantially opposite said anode, said cathode including at least one conductive carbon sheet [carbon] adapted to transfer electrons to said current collection component, said conductive carbon sheet disposed in electrolytic communication, via said electrolyte,

with said porous titania film,

and wherein said current collection component includes a graphite foil, disposed generally

in parallel with said conductive carbon sheet, said graphite foil adapted to collect current from said cathode.

47. The photovoltaic cell of claim 46, wherein said current collection component contains less than 2% binder, by weight.

48. The photovoltaic cell of claim 46, wherein said current collection component contains less than 1% binder, by weight.

49. The photovoltaic cell of claim 46, wherein said current collection component contains less than 0.5% binder, by weight.

50. The photovoltaic cell of claim 46, wherein said current collection component is substantially binderless.

51. The photovoltaic cell of claim 46, further comprising a metal conducting element, attached to said graphite foil, said metal conducting element extending through a side wall of the cell to effect current take-off.

52. The photovoltaic cell of claim 51, wherein said metal conducting element is a metal foil or a metal mesh.