The prior art relating to regenerative photovoltaic cells is crowded and diverse and, among others, possessed of detailed and precise knowledge concerning physical phenomena which can play a role within the framework of light conversion cells. EP-A- 0.333. 641 describes photo-electrochemical cells having a polycrystalline metal oxide semi- conductor with a monomolecular chromophore layer in its surface region whereby the metal oxide semi-conductor surface has a roughness factor of greater than 20, preferably greater than 150. At least one of the electrodes of the'641 reference shall be transparent to the incidental light. The photo-electrochemical cell in accordance with this reference is of the dual substrate "sandwich-type"configuration having oppositely charged current collectors attached to distinct substrates. The semi-conductor collector is provided with sintered nano-cells, such as TiO2.

Kavan et al., J. of The Electrochemical Society, February 1996, pages 394-400, have reported that sintering can reduce the (BET) surface area of nanocrystalline TiO2 and henceforth its photo-electrochemical conversion functionality. Péchy et al., J. Chem. Soc., Chem. Commun., 1995, pages 65-66, describe the preparation of phosphonated polypyridyl ligands which are reported to exhibit excellent charge-transfer sensitizer properties for nanocrystalline TiO2 film application. These characteristics render the phosphonated polypyridyl ligands particularly attractive for application in molecular photovoltaic devices.

Hanprasopwattana et al., Langmuir 1996,12, 3173-3179, describe methods of uniformly distributing monolayer to multilayer films of titania on mono-disperse silica spheres. The titania coatings so deposited are amorphous. Heating to temperatures around 500°C is needed to convert the amorphous coating into polycrystalline TiO2. Kay et al., The Journal of Physical Chemistry 1996, Vol. 98, pages 952-959, report on the mechanism of photosensitization of transparent TiO2 electrodes with chlorophyll derivatives, particularly induced photocurrent phenomena. Gratzel et al., Current Science, Vol. 66, No. 10,25 May 1994, describe properties and requirements of efficient dye-sensitized photoelectrochemical cells for direct conversion of visible light to current. The authors conclude that actual cell technology (at the date of the paper) is not sufficient and cannot lead to commercially viable executions.

Gratzel, DOE/BES workshop organized by NREL February 5-8,1996, Estes Park, CO, USA teaches that the price determining factor for photovoltaic technology originates from the conducting glass. In this context reference is drawn to the"monolith"construction of German patent 44 16 247 which exemplifies the construction of a sandwich-type structure, namely a cell wherein the photo sensitive layer is located between the current conductors thus necessarily requiring at least one transparent electrode. McEvoy et al., Solar Energy Materials and Solar Cells 32 (1994) 221-227, summarize general principles and the historical development sequence concerning photovoltaic technologies.

While significant efforts have been extended towards the development of photovoltaic cell technology which can be used commercially, these efforts have not yielded commercially viable executions. Past approaches were almost quantitatively directed to sandwich-type structures which are known to be difficultly manufacturable, in part because of constraints inherent to producing effective conductor areas.

It is therefore a main object of this invention to make available photovoltaic cells which can be manufactured economically and which are capable of efficiently converting incident light into electrical energy. It is another object of this invention to make available cell structures which do not require light transparent electrode arrangements and which can be manufactured economically and operated efficiently. It is still another object of this invention to provide photochemical cells or key components therefore which can be constructed on flexible materials.

This invention, in part, relates to a regenerative photovoltaic cell comprising two current collector areas, which are in ionic contact through an electrolyte, but electronically insulated from each other. One of the areas is made up of a conductive ion permeable porous, possibly fibrous woven or non woven material encapsulated by a dye coated semi-conductor functional layer, said semi-conductor layer facing direct exposure to incident light and being made of electronically-interconnected nano-cells that permit diffusion of electrolyte ions between the nano-cells, the other current collector area being physically separated by the electrolyte permeable electronically insulating material. The dye is capable of injecting electrons into the semi-conductor layer upon exposure to light having a wave-length in the range of from 300 nm to about 900 nm, more preferably from about 400 nm to about 700nm. The term"direct exposure"defines an arrangement whereby no photovoltaic (PV) functional element, such as conductive glass, conductive cover or grid, is present in between the semi-conductor comprising the electrolyte and the source of incident light.

The conductive porous material encapsulated by the semi conductor functional layer can be substantially constituted of natural or synthetic mineral, glass, carbon, conductive metals such titanium, tantalum, molybdenum, stainless steel (302; 304; 316; 321; 310; 17-7PH), Hastelloy C-276, tungsten, inconel, zirconium and comparable metals or metal alloys or combinations thereof well-known in the relevant domain. The terms: 302; 304; 316; 321; 310; 17-7PH; Hastelloy C-276; and inconel; have the meaning given in PERRY's CHEMICAL ENGINEERINGS'HANDBOOK, 66 Edition, R. H. Perry, D. W. Green, J. O.

Maloney, Me Graw Hill Book Company, 1984.

In a preferred execution, an electronically insulating but ion-permeable membrane or porous substrate can be used to separate the two electrodes. It (the substrate) can be glass, natural or synthetic mineral materials, natural or synthetic organic polymer including polyaramide, polyimide, polyester, polyether, polysulfone, polyketone, polyamide, epoxide, polyurethane, silicone, polyolefin and combinations thereof. This component can withstand without significant alterations temperatures up to 90 °C, preferably up to 120 °C. More in general, the substrate exhibits physical and chemical stability under conventional operating conditions.

The cathodic current collector can be represented by conductive paints and coatings, intrinsically conductive organic polymers, carbon or metals having established electrically conductive properties. Examples of suitable metals include titanium, tantalum, molybdenum, stainless steel (302; 304; 316; 321; 310; 17-7PH); Hastelloy C-276 and inconel. and comparable metals or metal alloys or combinations thereof well-known in the relevant domain. This layer shall be preferably made of a. an impermeable configuration such as a foil with a view to provide the photovoltaic construction with a degree of mechanical and environmental protection towards for instance ingression of water vapor or evaporation of the solvent used in combination with the electrolyte. However, the layer may also be constituted by a conductive porous material prepared along same principles as for the area supporting the dye coated semi conductor functional layer. A separate sheet of impermeable material shall then be needed to provide mechanical and environmental protection to the photovoltaic construction.

The counter electrode and its eventual protective sheet shall preferably withstand without significant alteration temperatures up to 120 °C.

The photovoltaic cell execution of this invention is preferably and generally covered by a transparent or translucent protective layer, such as glass or an appropriate transparent or translucent polymer, or multiple layers of polymer, which layer is substantially impermeable to water, organic vapors and oxygen. It also enables to significantly restrict exposure of the chromophore dye to ultraviolet light far shorter than 160 nm.. It should preferably withstand without significant alteration temperatures up to 120 °C. Preferred transparent or translucent polymers can be represented by polyesters, polyolefins, or combinations of polyesters with polyalkylenes having from 2 to 12, preferably 2 to 6 carbon atoms in the alkylene chain, or with polyfluoroalkenes having the same number of carbon atoms in the alkene chain.

The edges of the cell are carefully closed with preferably cross linkable sealants known in the art, such as silicone elastomers, epoxyde materials or polysulfides. The closure can also be achieved by heatwelding of the transparent and/or translucent polymers. The sealant should preferably withstand temperatures up to 90 °C, preferably up to 120 °C.

A cross-section of the photovoltaic cell of present invention is represented in Fig. 1.

The two current collector areas are electronically insulated in relation to each other. One area is in contact with/covered by a dye coated semi-conductor functional layer comprising electronically-interconnected nano-cells. This functional layer faces direct exposure to incident light. The other conductive layer can be represented by conductive surfaces represented by paints, coated surfaces, carbon, polymers or (pure) metals having established electrically conductive properties. Examples of suitable metals include titanium, nickel, tantalum, molybdenum, stainless steel (302; 304; 316; 321; 310; 17-7PH); Hastelloy C-276; inconel, chrome, platinum and comparable metals or metal alloys or combinations thereof well- known in the relevant domain. It can also be constituted of conductive porous materials along the principles applied in the preparation of the fabric supporting the dye coated semi conductor functional area.

The dye coated semi-conductor functional layer collects the electrons/electricity generated upon exposure to incident light. The semi-conductor functional layer comprises a structure capable of exhibiting semi-conductor or quasi-semi-conductor properties. The layer can be represented by a dye coated metaloxide semi-conductor. The semi- conductor component which is preferably characterized by a polycrystalline structure can be selected from transition metal oxides of an element selected from the fourth, fifth or sixth group or sub-group of the periodic system of elements, in particular titanium, zirconium, hafnium, strontium, zinc, indium, tin, antimony, yttrium, lanthanum, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, oxides of zinc, iron, nickel or silver, mixed oxides, and/or oxide mixtures of two or more of these metal oxides. In a preferred execution the semi-conductor layer comprises a polycrystalline TiO2 layer deposited onto a conductive layer, with anatase being the predominant and preferred crystalline TiO2 structure. The TiO2 semi-conductor layer can be prepared in accordance with the SOL-GEL process (Stalder and Augustynski, J. Electrochem. Soc. 1979,126, 2007) as applied in EP-A-0.333. 641, page 2, lines 48-56.

The semi-conductor layer is coated with a chromophore dye. The coating is preferably a monomolecular layer. The chromophore dye releases photoexcited electrons under the influence of incident light. The electrons originated from the photoexcited dye are irreversibly injected into the semi-conductor functional lattice and are consequently separated spatially and by a potential barrier from the oxidized dye coating. The spectral sensitivity can vary depending, among others, upon the chemical structure of the dye. The selection of a suitable dye can henceforth be optimized to thereby take into considerations relevant light parameters with a view to enhance the conversion of light to electrical energy. Chromophore dyes which can be used in regenerative photovoltaic cells are well known in the relevant domain. Suitable chromophore dyes can, for example, be represented by a transition metal complex salts such as for instance ruthenium (In trisbipyridyl salts or substituted ruthenium (In trissubstituted bipyridyl salts, ruthenium (II) bisbipyridyl complex salts or substituted ruthenium (II) disubstituted bipyridyl salts, osmium (II) trisbipyridyl salts or substituted osmium (II) trissubstituted bipyridyl salts or substituted osmium (II) trissubstituted bipyridyl salts.

An example of such chromophore is ruthenium cis-diaqua bis (2,2'-bipyridyl-4, 4')- dicarboxylate. The chromophore dye can also be an inorganic transition metal complex such as prussian blue. The chromophore dye can be attached to the surface of the semi-conductor by chemisorbtion, adsorption or comparable techniques capable of yielding adequate and economically viable conversion of light to electrical energy. It has been established that good results can be obtained in the event the chromophore dye is attached to the metal oxide semi- conductor by means of carboxylic acid ligand. The chromophore dye layer can also be represented by metal complexes of phthalocyanin or porphyrin.

The semi-conductor functional layer is impregnated with an electrolyte solution. The electrolyte participates regeneratively in the charge transport by acting as an electron donor in relation to the chromophore dye to thus compensate for the loss of photo- excited electrons which have irreversibly entered the semi-conductor functional layer e. g. the titanium dioxide polycrystalline particles deposited onto the conductive layer. The electrolyte solution for use within the photovoltaic cell herein can be represented by lithium iodide, bromide, hydroquinone and other comparable species which are well known in the relevant domain and have found application. The electrolyte is used in conjunction with a suitable solvent such as acetonitrile or similar organic nitriles and polynitriles. The electrolyte serves for the transport of electrical charges between the electrodes as for example described by Brian O'Regan et al., Nature, Vol. 353, pages 737-740.

Fig. 1 represents a cross-section of a photovoltaic cell execution of this invention having current collector areas on either side of a porous electronically insulating substrate. The cell 1 comprises a protective layer 2 which is substantially impermeable towards water, organic vapors and oxygen. This layer can be represented by flexible or rigid executions of metal, mineral or organic polymeric materials such as galvanized steel or glass.

The layer 3 comprises an anodic current collector area. It can be represented by a porous, possibly fibrous, woven or non woven material. Temperatures involved in the preparation of the semi conductor functional layer may go up to 500 C and materials capable of withstanding such temperatures are preferred. The porous material can be represented by conductive material or combinations of insulating and conductive materials.

Insulating material can be made for instance of a natural or synthetic mineral or of glass. The insulating porous material can be coated with a conductive material containing carbon, metals such as iron, chrome, nickel, tungsten, molybdenum, titanium, alloys of these metals, oxides of these metals, doped oxides of these metals or mixtures thereof. The conductive porous material can be represented by conductive materials including carbon, metals such as titanium, tantalum, molybdenum, stainless steel (302; 304; 316; 321; 310; 17-7PH;) Hastelloy C-276 tungsten, inconel, zirconium and comparable metals or metal-alloys and combinations thereof all well-known in the relevant domain. The conductive layer can also be constituted of randomly arranged conductive fibers in an insulating porous matrix, with preferably a proportion of conductive fibers with a ratio of length over diameter in excess of 10 (aspect ratio), preferably in excess of 30. The volume of conductive fibers shall be more than one percent of the space occupied by the porous mass (100%), preferably from 2 to 10%. The aspect ratio of the conductive fiber is preferably greater than 100, the fiber diameter is usually greater than 5, preferably from 5 to 100 micrometers.

A dye coated semi-conductor functional layer 4 comprising nanoparticles, prepared via methods described in prior art such as controlled hydrolysis of alkoxides followed by a heat treatment and interconnected through a suitable sintering process at temperatures up to 500 °C. The chromophore dye is applied to the interconnected nanoparticles.

While the semi-conductor component can be selected from a large variety of e. g. metal oxides, a polycrystalline titanium dioxide is a preferred execution. In another preferred aspect herein, a supplementary conductive layer substantially constituted of semi-conductor oxides or combinations thereof including, for example, oxides or fluorides of tin, indium, antimony or combinations thereof, is deposited upon the thermally resistant material before the deposition of the dye coated semi conductor functional layer 4 according to preparation procedure also known in the art such as controlled hydrolysis of related alkoxides followed by heat treatment at temperatures up to 500 °C. This supplementary layer facilitates and enhances the electrical connection of the anode current collector and the semi-conductor layer. The external semi- conductor functional layer is coated with a chromophore dye, preferably in a monomolecular layer.

The regenerative oxido. reduction couple electrolyte, which is respectively within the macroscopic and microscopic interstices/porosities of the porous materials constituting layers 3 and 7 of the semi-conductor functional layer 4, is preferably represented by lithium iodide and iodine in a molar ratio of from 10'to 1, preferably 20 to 5, dissolved in acetonitrile although other electrolyte and solvent species which have found application in comparable cell technology can also be used beneficially. The electrolyte solution has preferably a concentration in the range of from 1 molar to 104 molar, more preferably from about 10'molar to 103 molar.

A transparent protective layer 5 is deposited onto the semi-conductor functional, electrolyte impregnated, layer. The transparent material can be represented by glass and a variety of polymeric materials which are substantially impermeable to water, organic vapors and oxygen and limit exposure of the dye to far ultra violet light.

The counter electrode 6 is electronically isolated from the anodic current collector 3 and from the semi-conductor functional layer 4 by means of an ion-permeable insulating layer 7 consisting of a material such as a woven or non woven fabric, porous-, ceramic-, porcelain-, and/or polymeric coating material. The conductive material used in 6 can be selected independently from conductive materials suitable for use in layer 3. The counter electrode can also be executed by depositing conductive materials via conventional techniques well-known in the relevant domain on an appropriate substrate sheet. Examples of suitable metal deposition technologies include vacuum condensation, sputtering, electro-deposition, lamination, painting and combinations of such technologies. Minor quantities of oxido reduction catalysts such as platinum can also be beneficially applied on the surface of the electrode. Non- metallic conductive materials can be applied, depending upon the physical form of the conductive material and its coating properties. Metal and carbon paints can be applied by means of conventional techniques subject to optimized conditions. The non-metallic conductive material can broadly be represented by conductive coatings having preferably a conductivity of less than about 10 ohm/square. Such conductive materials can also be represented by a polymeric, intrinsically non-conductive matrix, containing a randomly dispersed minimal quantity, generally from about 1% to 15% of the volume expressed versus the matrix material (100%) of a high aspect ratio fibrous material, i. e. ratio of length over diameter, for example at least 10, generally more than 40, preferably more than 100, capable of conferring conductivity properties as may be required for application in the cell of this invention. The matrix material can be represented by polymer compounds including nylon, aramide, kevlar, polyesters and more generally any polymeric mass adapted and compatible to minor levels of materials capable of conferring conductivity properties. Examples of the like materials are fibrous carbon, metals, metal wires, metal meshes and screens, metal chips and metal powders and similar structures of insulating material coated by a conductive metal layer.

The edges of the photovoltaic cell are closed by means of a sealant electrically insulating materials 8 in a known manner. The insulating material can be represented by known materials such as crosslinkable natural or synthetic polymers, silicones or resins. The cured sealant shall be impermeable to water vapor, solvent vapor, iodine and oxygen.

The photovoltaic cell herein can be useful for converting incident light into electrical energy. Generally visible light which can be converted has a wavelength in the range of from 300 nm to around 900 nm. Daylight having a wavelength in the range of from 400 nm to 700 nm is for obvious reasons a prime source of light. It is understood, however, that any particular cell in accordance with this invention can be routinely adapted and optimized to any predominant source of visible light in the range of from 300 nm to 900 nm or more generally to any light source the radiation energy of which can be converted beneficially into electrical energy.

Examples: A slurry of titaniumdioxide nanoparticles was prepared as follows. 40 gs of TiO2 having an average particle diameter of 21 nm and a surface area in the range of from 35-65 m2 (Anatase; DEGUSSA P-25') was mixed with: 60 gs deionized water; 10 drops of a 10% aqueous nitric acid solution; 10 drops of non-ionic fluoroaliphatic polymeric ester supplied by 3M (Fluorad FC-430'); and 10 drops acetylacetone.

Example 1: A titanium gauze cell was prepared as follows. A piece of titanium gauze (50 mesh; 0.004 inch wire; 75 mm x 75 mm) was coated with the titanium dioxide slurry described above. After vacuum drying, the mesh was heated to 450 °C for 30 minutes, cooled to 80 °C and immersed into a ruthenium dye solution (21.15 mg RuL2 (SCN) 2-L = 4,4'-dicarboxy-2, 2'-bipyridyl-, CH3CN (50 mL) and t-BuOH (50 mL). After rinsing (CH3CN/t-BuOH) and drying at ambient temperature in a nitrogen stream, a cell was assembled using a 4 cm x 1 cm strip of the coated gauze. The counter electrode was a piece of gas diffusion electrode (Pt/C on nickel mesh) and the two electrodes were separated from each other by a piece of porous polypropylene membrane (supplied by 3M). Titanium wire made contact to each electrode and the assembly was heat sealed inside a transparent polyester"envelope"before adding electrolyte (0.5 M LiI/0. 05 M iodine in 3-methoxypropionitrile) with a syringe. The cell was then irradiated with a 500 watt photoflood bulb at a distance sufficient to produce a response equivalent to"one sun" on a calibrated silicon solar cell (Edmund Scientific).

An ammeter connected accross the two electrodes and a shortcircuit current of 28 mA and an open circuit voltage of 300 mV were measured.

Example 2: A tungsten gauze cell was made and tested as described in Example 1, except for the use of tungsten gauze (100 mesh; 0.002 inch wire; 1 cm x 5 cm). A shortcircuit current of 35 mA and an open circuit voltage of 414 mV were measured.

Example 3 : A Hastelloy cell was made and tested as described in Example 2, except for the use of Hastelloy C-276 mesh (50 mesh; 1 cm x 4 cm) A shortcircuit current of 11.5 mA and an open circuit voltage of 510 mV were measured.

Example 4: A stainless steel cell was assembled as described in Example 1 except for the use of a stainless steel 304 mesh (18 cm x 8 cm; 150 mesh) instead of titanium. The counter electrode was a piece of 316-stainless steel foil platinised by brushing with a 5 mM solution of H2PtCI6 in isopropanol and heating to 385 °C for 15 minutes. A shortcircuit current of 330 mA and an open circuit voltage of 414 mV were measured.

Example 5 : A stainless steel foil cell was made as follows. A piece of 316-stanless steel foil (5 cm x 1 cm) was perforated with small holes (diameter 0.5 mm; 5 mm spacing). This foil was coated and treated and assembled into a cell and tested as described in Example 1. A gas diffusion electrode was used as counter electrode. A shortcircuit current of 12.7 mA and an open circuit voltage of 323 mV were measured.

Example6: A nickel-plated glass fiber gauze cell was made as described in Example 1, except for the use on nickel-plated glass fiber woven gauze (150 mesh; nickel plating thickness 15 microns; 2 cm x 5 cm). The cell was irradiated with a 500 watt bulb at a distance sufficient to produce a response equivalent to"one tenth of a sun"as calibrated in accordance with Example 1. A short circuit current of 2.1 mA and an open circuit voltage of 485 mV were measured.