WO2009053979A2

WO2009053979A2 - Monolithic dye cell assembly having reduced ion migration in intercell seals

Info

Publication number: WO2009053979A2
Application number: PCT/IL2008/001399
Authority: WO
Inventors: Jonathan R. Goldstein
Original assignee: 3Gsolar Ltd.
Priority date: 2007-10-24
Filing date: 2008-10-23
Publication date: 2009-04-30
Also published as: WO2009053979A3

Abstract

A monolithic photovoltaic dye cell assembly and method for achieving reduced ion migration through at least one intercell junction, the assembly including: (a) an electrically insulating transparent substrate; (b) a plurality of photovoltaic units arranged as parallel strips on the substrate, each unit including: (i) a transparent conductive coating disposed on the substrate; (ii) a photoanode including: (A) a porous layer of a polycrystalline semiconductor, disposed on the conductive coating, and (B) a dye, absorbed on a surface of the porous layer, the dye and the porous layer adapted to convert photons to electrons; (iii) a cathode, disposed opposite the photoanode, and including a conductive carbon layer, (iv) a porous separator layer interdisposed between the porous layer of the photoanode and the cathode; (v) an electrolyte, disposed within each of the photovoltaic units, and containing a redox species; (c) a sealant layer, at least partially disposed between the adjacent units of the photovoltaic units to electrically insulate therebetween, the conductive carbon layer disposed in electrolytic communication, via the electrolyte, with the porous layer, and (d) an ion-migration reduction system including first and second electrodes, electrically connected to a current source and forming an electrode pair, and disposed within the electrically-insulating sealant layer, the ion-migration reduction system adapted to deliver a potential between the electrodes.

Description

Monolithic Dye Cell Assembly Having Reduced Ion Migration in Intercell Seals

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, and more particularly, to a monolithic photovoltaic dye cell assembly having reduced ion migration in intercell seals, and a method of operating such a dye cell assembly.

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 and coated with a light sensitive dye.

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 including nanocrystalline titania, a carbon counter-electrode (cathode), which is a porous, catalytic, electrically conducting carbon-based structure bonded together using a titania binder, and an intermediate electrically insulating porous layer based on alumina, silica, titania or zirconia powder, for separating the anode from the cathode. The pores of the intermediate layer are at least partially filled with a liquid phase, ion-transferring electrolyte, following coating of the nanocrystalline titania with a light-sensitive dye. A current collecting layer of a tin oxide based transparent, electrically-conducting material is situated between the transparent substrate and the anode. The anode and cathode of a given cell provide a direct-current voltage when the anode is exposed to light, such that series assemblies of cells may readily be built up. The cathode of each succeeding element is connected with the intermediate conducting

A layer of the preceding anode element, 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 micron) 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 microns thick (in contact with the tin oxide layer) based on titania, having a nanocrystalline characteristic particle size of 10-50 nm, 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) are appropriate for inclusion as conductors as part of the photoanode, due to the rigorous criteria for the conductor chosen, including chemical inertness to the electrolyte, specific electrical resistivity below 10^"4 ohm^»cm, and characteristically no tendency, or practically no tendency for recombination.

For cells that are partially transparent, the other electrode (the counter-electrode) includes a thin layer of catalyst (usually containing a few micrograms of platinum per square 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 with no vapor pressure, and even different redox species, may be used, however. U.S. Patent No. 5,350,644 to Graetzel, et al., discloses various dye cell chemistries, especially different dyes based on ruthenium complexes. Photons falling on the photoelectrode excite the dye (creating activated oxidized dye molecules), causing electrons to enter the conduction band of the titania and to flow (via an outer circuit having a load) to the counter-electrode. There, the electrons reduce 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/square 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 stated 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.

It should be noted that in the above-referenced patent to Kay, adequate sealing between adjacent cells so as to effectively prevent any ion migration is not addressed, and remains a serious unanswered challenge. We have found that ion migration of iodine species between cells may reduce the iodine inventory in cells and appreciably limit the lifetime of monolithic assemblies.

SUMMARY OF THE INVENTION

According to the teachings of the present invention there is provided a monolithic photovoltaic dye cell assembly having reduced ion migration through at least one intercell junction, the assembly including: (a) an electrically insulating transparent substrate having a first surface; (b) a plurality of photovoltaic units arranged as substantially parallel strips on the surface, each unit of the photovoltaic units including: (i) an at least partially transparent conductive coating disposed on the surface; (ii) a photoanode including: (A) a porous layer of a polycrystalline semiconductor, disposed on the conductive coating, and (B) a dye, absorbed on a surface of the porous layer, the dye and the porous layer adapted to convert photons to electrons; (iii) a cathode, disposed substantially opposite the photoanode, the cathode including a conductive carbon layer; (iv) a porous separator layer interdisposed between the porous layer of the photoanode and the cathode, the porous separator layer adapted to physically separate and electrically insulate between the porous layer of the photoanode and the cathode, and (v) an electrolyte, disposed within each of the photovoltaic units, the electrolyte containing a redox species, the coating being discontinuous with respect to adjacent units of the photovoltaic units; (c) a sealant layer, at least partially disposed between the adjacent units of the photovoltaic units to electrically insulate between the adjacent units, the conductive carbon layer disposed in electrolytic communication, via the electrolyte, with the porous layer of the polycrystalline semiconductor, the plurality of photovoltaic units and the transparent substrate forming a monolithic assembly, and (d) an ion-migration reduction system including at least a first electrode and a second electrode, electrically connected to a current source and forming an electrode pair, and disposed within the electrically- insulating sealant layer, the ion-migration reduction system adapted to deliver a potential from the first electrode to the second electrode, wherein at least the first electrode is disposed between the adjacent units of the photovoltaic units.

According to another aspect of the present invention there is provided a method of reducing ion migration through at least one intercell junction of a monolithic photovoltaic dye cell assembly, the method including the steps of: (a) providing the monolithic photovoltaic dye cell assembly, the assembly including: (i) an electrically insulating transparent substrate having a first surface; (ii) a plurality of photovoltaic units arranged as substantially parallel strips on the surface, each unit of the photovoltaic units including: (A) an at least partially transparent conductive coating disposed on the surface; (B) a photoanode including: (I) a porous layer of a polycrystalline semiconductor, disposed on the conductive coating, and (II) a dye, absorbed on a surface of the porous layer, the dye and the porous layer of the polycrystalline semiconductor adapted to convert photons to electrons; (C) a cathode, disposed substantially opposite the photoanode, the cathode including a conductive carbon layer; (D) a porous separator layer interdisposed between the porous layer of the photoanode and the cathode, the porous separator layer adapted to physically separate and electrically insulate between the porous layer of the photoanode and the cathode, and (E) an electrolyte, disposed within the photovoltaic units, the electrolyte containing a redox species, the coating being discontinuous with respect to adjacent units of the photovoltaic units; (iii) a sealant layer, at least partially disposed between adjacent units of the photovoltaic units to electrically insulate between the adjacent units, the conductive carbon layer disposed in electrolytic communication, via the electrolyte, with the porous layer of the polycrystalline semiconductor, the plurality of photovoltaic units and the transparent substrate forming a monolithic assembly, and (iv) an ion- migration reduction system including at least a first electrode and a second electrode, electrically connected to a current source and forming an electrode pair, and disposed within the electrically-insulating sealant layer, the ion-migration reduction system adapted to deliver a potential from the first electrode to the second electrode, wherein at least the first electrode is disposed between the adjacent units of the photovoltaic units, and (b) delivering the potential from the first electrode to the second electrode to reduce ion migration through the intercell junction.

According to further features in the described preferred embodiments, the first electrode is closer to the transparent conductive coating than the second electrode, and the potential from the first electrode to the second electrode is adapted to match an electrical sign of ions of the redox species.

According to still further features in the described preferred embodiments, the first electrode includes a conductive element at least partially enveloped by a porous enveloping insulating layer.

According to still further features in the described preferred embodiments, the porous enveloping insulating layer is juxtaposed against the transparent, electrically conductive layer. According to still further features in the described preferred embodiments, the first electrode is disposed within 20 micrometers of the transparent, electrically conductive layer.

According to still further features in the described preferred embodiments, the first electrode is disposed within 10 micrometers of the transparent, electrically conductive layer.

According to still further features in the described preferred embodiments, the photovoltaic units are disposed in an electrical series.

According to still further features in the described preferred embodiments, the current source is electrically external to the monolithic dye cell assembly. According to still further features in the described preferred embodiments, the potential is produced within the monolithic dye cell assembly.

According to still further features in the described preferred embodiments, a magnitude of the potential is at least 60%, or at least 80%, of an open-circuit potential of a single one of the photovoltaic units. According to still further features in the described preferred embodiments, a magnitude of the potential is at least 80%, or at least 100%, of an operating potential of a single one of the photovoltaic units.

According to still further features in the described preferred embodiments, a magnitude of the potential is at least 30%, at least 50%, or at least 70% of a combined operating potential of all the photovoltaic units disposed in series.

According to still further features in the described preferred embodiments, the redox species is an iodine-based redox species.

According to still further features in the described preferred embodiments, the redox species includes a cobalt complex.

According to still further features in the described preferred embodiments, a magnitude of the potential is at least 1.5 Volts, at least 3 Volts, or at least 10 Volts.

According to still further features in the described preferred embodiments, the assembly is covered by a liquid-tight cover.

According to still further features in the described preferred embodiments, the plurality of photovoltaic units includes a first photovoltaic unit, a last photovoltaic unit, and at least one intermediate photovoltaic unit, the first, last, and intermediate units disposed in an electrical series. According to still further features in the described preferred embodiments, the polycrystalline semiconductor includes titanium dioxide.

According to still further features in the described preferred embodiments, the porous separator layer includes zirconia.

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 a monolithic dye cell assembly of the prior art;

Figure 2 is a schematic, expanded cross-sectional view of a junction between two adjacent cells of a monolithic multiple dye cell assembly according to the present invention, in which junction are disposed a pair of electrodes, and

Figure 3 provides a schematic, partially cut-open representation of one preferred embodiment of the inventive multiple-cell module, in which the intercell junctions are adapted to receive an imposed voltage to prevent or reduce intercell ion migration.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

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

The present invention provides a means for prevention or reduction of ion migration through intercell seals of monolithic dye cell assemblies. A cross-sectional view of a prior art monolithic dye cell assembly of U.S. Patent No. 6,069,313 to Kay is shown in Figure 1. A glass substrate 1 is provided having a substantially transparent, electrically conductive layer 2, usually based on tin oxide. The conductive layer is cut in a set of parallel lines at equidistant spacing across the face of the conducting substrate, usually by laser grooving means, so as to provide a series of narrow, parallel, conductive strips 2a, 2b, 2c, several millimeters wide, along the length of substrate 1, but electrically insulated from each other on the substrate by electrically insulated regions 3.

A layer of porous nanocrystalline titania 4 is screen-printed on each of the strips to act as photoanode, followed by a porous insulating layer 5 based on zirconia, silica, alumina or titania. A counter-electrode 6 of the cell, which is printed over the groove so as to make electrical contact with the adjacent conducting strip, consists of a porous layer of carbon containing a suitable binder such as titania (and optionally containing a precious metal catalyst), which is screen-printed on top of an insulator layer 5 in each individual cell 150, 160, 170. This arrangement gives a series connection of adjacent cells on a single substrate (monolithic assembly) without the need for additional cell interconnection means.

Printing, drying and sintering schedules for layers 4, 5, 6 can be preferably designed to involve a single sintering step for the layers. Typical thicknesses for the sintered layers are 15 microns for the titania, 5 microns for the insulator, and 50 microns for the counter-electrode (carbon). Dye is introduced into the titania layer of all the cells by printing application of a solution of dye in an organic solvent onto the porous carbon layer of each cell, following which this solution flows down to the titania layer, where dye is absorbed. Following evaporation off of the dye solvent, electrolyte is introduced into the cells by printing application onto the porous carbon layer in each cell. A monolithic series stack, assembly, or multiple-cell module 100 is made up of a plurality of single cells such as individual cells 150, 160, 170, connected in series. Each of individual cells 150, 160, 170 includes a titania layer 4, insulating layer 5 disposed thereon, and a counter-electrode 6 disposed generally above insulating layer 5. A sealant layer such as polymer sealant layer or coating 7 is disposed so as to electrically seal adjacent cells (e.g., individual cells 150 and 160) one from another, and to cover the large area face of series stack 100. Additional sealing protection is achieved by bonding on top of the polymer a sheet 8 of glass, plastic or laminate. Voltage takeoff from the series stack 100 is via bus bars 9, 10 situated at the extremities of stack 100 (bus bar 9 for the positive pole and bus bar 10 for the negative pole). Bus bars 9, 10 can be based on silver or solder, by way of example, since these structural elements are external to the cell electrolyte and are not exposed to the corrosive conditions.

The above-described monolithic assembly suffers from inherent lifetime limitations. A maximum width for the titania strips of about one centimeter is typical, since this gives an acceptable ohmic loss from the cell under practical operating conditions and as dictated by the limited electrical conductivity of the tin oxide glass and the carbon counter-electrode. In order to achieve an acceptable minimal shading (of 10%, by way of example) from such strip assemblies, the polymer sealant layer between adjacent cells cannot be more than one millimeter thick. Known polymer sealants may be adequate for sealing between the cells and bonding to the tin oxide glass, however, such sealants cannot, at such a narrow spacing, sufficiently resist iodine migration between adjacent cells for more than several months. This is unacceptable in view of the lifetime requirements for solar cells. The problem is compounded by the fact that the strong potential field (about

700mV/cell) that exists between adjacent cells can drive the iodine migration. The iodine, in the form of the iodide ion I^" or the tri-iodide ion I₃ ^" _, is a critical component of the cell electrolyte since it comprises the redox couple on which the operation of the cell depends. The iodine inventory of a cell (the total iodine present) and the iodine concentration in the electrolyte are well-defined parameters operating between well- defined limits for optimum cell performance. Any loss of iodine from the inventory of a particular cell (by migration to the adjacent cell) will cause a fall-off in performance of that cell, and this poorly performing cell will limit the performance of the entire stack. In particular, we have found that the iodine migration tends to occur along the surface of the tin oxide layer at the very base of the sealant. Iodine migration may also occur through the bulk sealant itself. Improved sealing techniques and materials combinations, or successive application of layers of different adhesives, may increase material and fabrication costs of the cell, without providing the requisite long-term protection against iodine migration. The present invention uses an imposed voltage field to prevent or reduce iodine migration between adjacent cells. Figure 2 shows an expanded side view (not to scale) of the junction between two adjacent cells shown in Figure 1. Within polymer sealant layer 7 between adjacent cells are embedded two electrodes, i.e., conductive elements 20 and 25, typically in the form of wires or strips. Each of these conductive elements may be coated with a porous, insulating coating 30 to prevent short-circuiting (between wires and/or between wires and transparent, electrically conductive layer 2).

Wire 20 (preferably within coating 30) is positioned close to, or juxtaposed against, transparent, electrically conductive layer 2 on glass substrate 1. Wire 25 is disposed at a larger distance than wire 20, with respect to conductive layer 2. In Figure 2, wire 25 is situated substantially above wire 20, and within the intercell junction defined as the region between adjacent photovoltaic units (e.g., between counter- electrode 6 associated with conductive strip 2a, and insulating layer 5 and counter- electrode 6 associated with conductive strip 2b). A voltage is applied across the electrodes such as wires 20 and 25 by means of a direct current source. The direct current source may be the photovoltaic assembly itself, as shown hereinbelow in Figure

3. The direct current source may include an external direct current source (not shown). ^"

In the case of an iodine-based, or other negatively charged, redox species, the voltage is applied such that wire 20 has a negative polarity. Similar wire couples under a similarly applied voltage field are embedded in the other intercell junctions, as shown in Figure 3 hereinbelow.

The electrodes, such as conductive elements 20 and 25 may be made of an iodine-resistant material such as tungsten, titanium, stainless steel or titanium-clad copper. The specific electrical resistivity of the conductive elements is preferably below, or well below, 10^"4 ohm»cm. The insulating coating is porous, and may include, by way of example, zirconia, silica, alumina or titania, applied by a coating procedure based on a suitable process, for example by dip-coating or electrophoresis followed by drying and baking. The insulating coating may also be any porous material resistant and preferably substantially inert to chemical attack of the electrolyte. Such insulating coatings may include polymeric and/or fibrous materials. A narrow gauge of wire and coating (for example 50-150 microns for the wire and 10-30 microns for the coating) may enable secure embedding in the polymer sealing between cells without necessarily increasing the seal width and shading. Each wire or electrode pair is supplied with a direct current voltage or potential, as described above. The voltages may be supplied by the illuminated monolithic dye cell module itself, but it is also possible to provide the voltage using a battery or by other means known to those of ordinary skill in the art. In a typical monolithic dye cell module of size 30cmx30cm, and assuming each cell element is lcm wide with an operating voltage of 600mV/cell, there is about 18V available to be applied across the wires of each intercell junction. Leads or connections are preferably provided so as to connect the module bus bars (shown in Figure 1) to each of the embedded electrode pairs 20, 25.

Figure 3 shows, in schematic form, a preferred embodiment of a multiple-cell module such as a 3-cell module or stack 40 having cells I, II, and III, adapted so as to prevent or appreciably reduce intercell ion migration. The details of the cell structure of may be essentially as shown in Figures 1 and 2, and as described in the description associated therewith. Bus bars 50 and 55 provide the main output of power from the module under illumination, via leads such as regular heavy duty leads 60 and 65. However, from (negative pole) bus bar 50 extend two additional, typically low power leads 52 and 54, which are electrically connected, respectively, to embedded electrode or wire 90 in a junction 72 (whose volume is at least partially filled by sealant coating 7, as described with respect to Figure 2) between cell I and cell II, and to embedded electrode or wire 95 in a junction 77 (whose volume, like that of junction 72, is at least partially filled by sealant coating 7 (referenced hereinabove) between cell II and cell III, these wires being positioned closest to the transparent, conductive (e.g., tin oxide coating) layer in the cell. Similarly, from (positive pole) bus bar 55 extend two additional, typically low power leads, which are electrically connected, respectively, to embedded electrode or wire 80 in junction 77 between cells II and III, and to embedded electrode or wire 85 in junction 72 between cells I and II. Wires 80, 85 are preferably positioned relatively far (with respect to embedded wires 90 and 95) from the tin oxide layer in the cells, so as to ensure the requisite polarity for inhibiting the ion migration. The connection scheme provides a high voltage field and high negative potential at the intercell junctions close to the tin oxide surface and prevents or retards iodine migration across the interfaces.

The ion-migration reduction system of the present invention includes a current source such as a monolithic photovoltaic dye assembly, or an external current source (such as a battery or DC current source), and at least one electrode pair such as electrodes 80, 90, disposed within a sealant layer such as sealant coating 7, preferably within an intercell junction such as junction 72, the electrode pair being electrically connected to the current source.

With reference now to both Figures 2 and 3, the magnitude of the negative potential is preferably adapted so as to inhibit or significantly reduce migration of iodide and tri-iodide ions (which are both negatively charged) across a sealant layer filled junction such as junction 72 and into the adjacent cell. More generally, the magnitude of the potential is preferably adapted so as to inhibit or significantly reduce migration of the redox ions (which may be positively charged or negatively charged, depending on the system) across sealant layer 7 (filling a junction such as junction 72) and into the adjacent cell.

Preferably, the magnitude of the applied potential is at least 60% of the open- circuit potential of a single cell such as cell I, or at least 80% of the operating potential of such a cell. More preferably, the magnitude of the applied potential is at least 80% of the open-circuit potential of a single cell such as cell I₅ or more typically, at least about 100% of the operating potential of such a cell.

The requisite magnitude of the applied potential may be related to the operating potential of the entire stack. Preferably, the magnitude of the applied potential is at least 30% of the operating potential of the entire stack or module such as module 40, yet more preferably, at least 50% of the operating potential of the entire module, and most preferably, at least 70% of the operating potential of the entire module.

Given that bus bars 50 and 55 are often advantageously disposed at the extremities of module 40, practically the magnitude of the applied potential may be substantially equal to the operating potential of the entire module.

In terms of absolute potential, the applied voltage is preferably at least about 1.5 Volts, and more preferably, at least about 3 Volts. Higher voltages (e.g., above 5 Volts, above 10 Volts, above 20 Volts) tend to improve the reduction of ion migration. It will be apparent to one skilled in the art that redox species other than iodine- based species may be used in conjunction with the present invention. For example, in the Journal of the American Chemical Society, Volume 124, page 11215 (2002), Elliot describes the use of cobalt II/III complexes as an alternative to the iodide/tri-iodide redox couple in dye cells, since they are less corrosive and less volatile than the iodine- based systems. In the cobalt system, both redox species have a positive charge.

If such a cobalt based redox system were to be used in monolithic cell assemblies according to the present invention, the imposed potential would be configured such that the positive lead having the higher potential would be embedded at the base of the intercell seal close to the tin oxide layer, thereby limiting or preventing migration of the positively charged redox species between adjacent cells.

As used herein in the specification and in the claims section that follows, the terms "positive potential" and "negative potential", with regard to an electrode pair, are used in a relative sense, and not necessarily in an absolute sense.

Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. AU publications, patents and patent applications mentioned in this specification, and in particular, U.S. Patent No. 5,350,644 to Graetzel and U.S. Patent No. 6,069,313 to Kay, are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application 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 monolithic photovoltaic dye cell assembly having reduced ion migration through at least one intercell junction, the assembly comprising:

(a) an electrically insulating transparent substrate having a first surface;

(b) a plurality of photovoltaic units arranged as substantially parallel strips on said surface, each unit of said photovoltaic units including:

(i) an at least partially transparent conductive coating disposed on said surface;

(ii) a photoanode including:

(A) a porous layer of a polycrystalline semiconductor, disposed on said conductive coating, and

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

(iii) a cathode, disposed substantially opposite said photoanode, said cathode including a conductive carbon layer;

(iv) a porous separator layer interdisposed between said porous layer of said photoanode and said cathode, said porous separator layer adapted to physically separate and electrically insulate between said porous layer of said photoanode and said cathode, and

(v) an electrolyte, disposed within each of said photovoltaic units, said electrolyte containing a redox species, said coating being discontinuous with respect to adjacent units of said photovoltaic units;

(c) a sealant layer, at least partially disposed between said adjacent units of said photovoltaic units to electrically insulate between said adjacent units, said conductive carbon layer disposed in electrolytic communication, via said electrolyte, with said porous layer of said polycrystalline semiconductor, said plurality of photovoltaic units and said transparent substrate forming a monolithic assembly, and

(d) an ion-migration reduction system including at least a first electrode and a second electrode, electrically connected to a current source and forming an electrode pair, and disposed within said electrically-insulating sealant layer, said ion-migration reduction system adapted to deliver a potential from said first electrode to said second electrode, wherein at least said first electrode is disposed between said adjacent units of said photovoltaic units.

2. The monolithic dye cell assembly of claim 1, wherein said first electrode is closer to said transparent conductive coating than said second electrode, and wherein said potential from said first electrode to said second electrode is adapted to match an electrical sign of ions of said redox species.

3. The monolithic dye cell assembly of claim 1, wherein said first electrode includes a conductive element at least partially enveloped by a porous enveloping insulating layer.

4. The monolithic dye cell assembly of claim 3, wherein said porous enveloping insulating layer is juxtaposed against said transparent, electrically conductive layer.

5. The monolithic dye cell assembly of claim 1, wherein said first electrode is disposed within 20 micrometers of said transparent, electrically conductive layer.

6. The monolithic dye cell assembly of claim 3, wherein said first electrode is disposed within 10 micrometers of said transparent, electrically conductive layer.

7. The monolithic dye cell assembly of claim 1, wherein said photovoltaic units are disposed in an electrical series.

8. The monolithic dye cell assembly of claim 2, wherein said current source is electrically external to the monolithic dye cell assembly.

9. The monolithic dye cell assembly of claim 2, wherein said potential is produced within the monolithic dye cell assembly.

10. The monolithic dye cell assembly of claim 1, wherein a magnitude of said potential is at least 60% of an open-circuit potential of a single one of said photovoltaic units.

11. The monolithic dye cell assembly of claim 1 , wherein a magnitude of said potential is at least 80% of an open-circuit potential of a single one of said photovoltaic units.

12. The monolithic dye cell assembly of claim 1, wherein a magnitude of said potential is at least 80% of an operating potential of a single one of said photovoltaic units.

13. The monolithic dye cell assembly of claim 1, wherein a magnitude of said potential is at least 100% of an operating potential of a single one of said photovoltaic units.

14. The monolithic dye cell assembly of claim 7, wherein a magnitude of said potential is at least 30% of a combined operating potential of all said photovoltaic units.

15. The monolithic dye cell assembly of claim 7, wherein a magnitude of said potential is at least 50% of a combined operating potential of all said photovoltaic units.

16. The monolithic dye cell assembly of claim 7, wherein a magnitude of said potential is at least 70% of a combined operating potential of the monolithic dye cell assembly.

17. The monolithic dye cell assembly of claim I₅ wherein said redox species is an iodine-based redox species.

18. The monolithic dye cell assembly of claim 1, wherein said redox species includes a cobalt complex.

19. The monolithic dye cell assembly of claim I₅ wherein a magnitude of said potential is at least 1.5 Volts.

20. The monolithic dye cell assembly of claim I₅ wherein a magnitude of said potential is at least 3 Volts.

21. The monolithic dye cell assembly of claim 1, wherein a magnitude of said potential is at least 10 Volts.

22. The monolithic dye cell assembly of claim I₅ the assembly being covered by a liquid-tight cover.

23. The monolithic dye cell assembly of claim I₅ wherein said plurality of photovoltaic units includes a first photovoltaic unit, a last photovoltaic unit, and at least one intermediate photovoltaic unit, said first, last, and intermediate units disposed in an electrical series.

24. The monolithic dye cell assembly of claim 1, wherein said polycrystalline semiconductor includes titanium dioxide.

25. The monolithic dye cell assembly of claim 1, wherein said porous separator layer includes zirconia.

26. A method of reducing ion migration through at least one intercell junction of a monolithic photovoltaic dye cell assembly, the method comprising the steps of:

(a) providing the monolithic photovoltaic dye cell assembly, the assembly including:

(i) an electrically insulating transparent substrate having a first surface;

(ii) a plurality of photovoltaic units arranged as substantially parallel strips on said surface, each unit of said photovoltaic units including:

(A) an at least partially transparent conductive coating disposed on said surface;

(B) a photoanode including:

(I) a porous layer of a polycrystalline semiconductor, disposed on said conductive coating, and

(II) a dye, absorbed on a surface of said porous layer, said dye and said porous layer of said polycrystalline semiconductor adapted to convert photons to electrons;

(C) a cathode, disposed substantially opposite said photoanode, said cathode including a conductive carbon layer;

(D) a porous separator layer interdisposed between said porous layer of said photoanode and said cathode, said porous separator layer adapted to physically separate and electrically insulate between said porous layer of said photoanode and said cathode, and

(E) an electrolyte, disposed within said photovoltaic units, said electrolyte containing a redox species, said coating being discontinuous with respect to adjacent units of said photovoltaic units;

(iii) a sealant layer, at least partially disposed between adjacent units of said photovoltaic units to electrically insulate between said adjacent units, said conductive carbon layer disposed in electrolytic communication, via said electrolyte, with said porous layer of said polycrystalline semiconductor, said plurality of photovoltaic units and said transparent substrate forming a monolithic assembly, and

(iv) an ion-migration reduction system including at least a first electrode and a second electrode, electrically connected to a current source and forming an electrode pair, and disposed within said electrically-insulating sealant layer, said ion-migration reduction system adapted to deliver a potential from said first electrode to said second electrode, wherein at least said first electrode is disposed between said adjacent units of said photovoltaic units, and

(b) delivering said potential from said first electrode to said second electrode to reduce ion migration through the intercell junction.