The present invention concerns vertical electrochemical contacts of photoelectrochemical cells or DSSC (dye-sensitized solar cells) with low visual impact.

More in detail, the invention concerns the structure of said vertical electrochemical contacts, integrated into the photovoltaic modules of DSSC cells, aimed primarily at reducing the visual impact of such contacts, but also to an improvement of their operating performance, and a process for their realisation.

In the context of the present invention, by the term "electrochemical contact" is meant a contact obtained through an electrolytic solution with a redox shuttle.

DSSC cells are photovoltaic cells consisting of a multilayer structure delimited by two substrates. Typically, said substrates are made of transparent materials (preferably glass, but also PET or PEN) and are coated, on the side facing the interior of the multilayer structure, by an electrically conductive coating which is also transparent (usually a transparent conductive oxide, preferably a titanium oxide doped with fluorine or iodine, respectively FTO and ITO).

Between the two substrates are arranged a photo-electrode (the anode), positioned on the conductive coating of one of the two substrates, a counter-electrode (the cathode), positioned on the conductive coating of the other substrate, and an electrolyte interposed between said photo- electrode and said counter-electrode. In particular, the photo-electrode is usually made of a porous titanium oxide, that supports the active material, consisting of a dye capable of transferring electrons following the absorption of a photon. The counter-electrode is usually made of platinum, while the electrolytic solution is generally based on iodine ( ) and potassium iodide (Kl).

Photoelectrochemical cells of this type have been described for example in U.S. Patent No. 4,927,721 ; materials usable in this type of cells have been described for example in U.S. Patent No. 5,350,644.

By their nature, the conductive coatings of the structures have high resistances. In addition, individual cells of this type are not able to generate the voltage levels required in most of the possible applications where a photoelectrochemical cell can be addressed.

To overcome these drawbacks, it is therefore necessary to connect a plurality of photoelectrochemical cells in series with each other, with the result of generating higher voltage differences minimizing the total current, ie minimizing power losses due to the resistance of the conductive coatings.

In practice, on the same substrate is realised a photoelectrochemical module, ie the conductive coatings of each substrate are divided into a plurality of electrically insulated regions, generally shaped as a plurality of stripes side by side, each region of the conductive coating of one of the two substrates being arranged in a position coinciding and only slightly offset in the transverse direction compared to that of a region of the conductive coating of the other substrate, a cell being realised between each pair of coincident regions of the two substrates. The side by side photoelectrochemical cells, thus obtained, are connected in series with a connection integrated on the same substrate, realised during the making of the module.

The connections in series integrated on the substrate can be made according to different schemes, known as Z connection, W connection and external connection.

The connections of type Z are constituted by a series of vertical contacts, arranged in the space between two adjacent cells, in particular in the space between the long sides of two cells, ie in the space not used for the cell by reason of the offset arrangement of the regions electrically insulated from the conductive coatings of the two substrates, and which connect with each other regions of the electrically insulated conductive coating of the two substrates, according to a configuration which will be explained in greater detail in the following description.

The connections of type W are obtained without need of contacts, but the configuration of the resulting photoelectrochemical module tends to have internal current mismatches, since the half of the cells in a module with this configuration are illuminated from the side of the counter- electrode . Moreover, on the same substrate photo-electrode and counter- electrode are alternated: titanium dioxide and platinum are then deposited and sintered simultaneously. This implies the inability to individually optimize the sintering process of the two materials, which normally have different optimal temperatures and curing times (420 °C and approximately 15 minutes for platinum; 500 °C and approximately 30 minutes for titanium dioxide ).

As for the external connection, however, this type of connection is realised on the edge of the module, in the vicinity of the short sides of the stripes that form the photoelectrochemical cells. The long path which the electrons must do to exit the sides of the module, where the connection occurs between individual cells, limits the length of the cells (to avoid that they can add additional losses due to the resistances) and greatly impacts on the fill factor of the module, characteristic parameter which describes the relationship between the maximum power produced by the device and the product of open circuit voltage multiplied by the closed circuit current, and that decreases proportionally to the increase of the value of the resistance introduced by the connections between cells and by the resistances of loss introduced by long paths of electrons.

With reference to Figure 1 , there is shown schematically the configuration of a connection of type Z between two cells of a photoelectrochemical module seen in a section transverse to the direction of the stripes defined by the photoelectrochemical cells.

In particular, Figure 1 shows the two substrates, indicated with the reference numeral 10, each of which is coated, on the side facing the other substrate, by a transparent electrically conductive coating 11. The conductive coating 11 is divided into electrically insulated regions by means of interruptions 12. Each photoelectrochemical cell is made in the area between two overlapping electrically insulated regions of conductive coatings of the two opposing substrates, each cell being comprised of a photo-electrode 13, positioned on the conductive coating 11 of one of the two substrates 10, a counter-electrode 14, positioned on the conductive coating 11 of the other substrate 10, and a liquid electrolyte interposed between said photo-electrode 13 and said counter-electrode 14.

Each cell is delimited laterally by an encapsulant 16, which serves to retain the liquid electrolyte inside the cell.

The connection in series between the two cells is obtained by means of the vertical electrical contact 17, which connects the offset portion of the electrically insulated region of the conductive coating 11 of one of the two substrates 10 with the coincident staggered portion in the electrically insulated region of the conductive coating 11 of the opposite substrate 10.

The connection path by means of the vertical contact can be schematized by three resistors: a first resistor constituted by the contact resistance between the conductive coating 11 disposed on the first substrate 10 and the vertical electrical contact 17, a second resistor formed by the resistance of the material of the contact vertical electric 17 itself and a third resistor formed by the contact resistance between the electrical contact 17 and the vertical conductive coating 11 disposed on the substrate 10 opposite the first.

According to the prior art, the vertical electrical contact 17 can be realised by means of different technologies:

- depositing a conductive paste on the conductive coating on both substrates and sintering of the same paste prior to mating of the substrates to form the photoelectrochemical module (encapsulation);

- depositing a conductive paste on the conductive coating of a single substrate and sintering of the same prior to mating of the substrates to form the photoelectrochemical module (encapsulation), or

- depositing a conductive paste (on the conductive coating of one or both of the substrates) and curing the same in the sealing stage of the photoelectrochemical module.

The connections thus realised, however, have problems of electrical conduction with increasing temperature. This is due to different thermal behavior between the material that constitutes the electric contact and the material of the encapsulant that maintains the liquid electrolyte within the respective cells.

Furthermore, the connections of this type have not optimal conductivity values, besides the problem of the degradation of its performance with increasing temperature.

But above all, the connections of this type are extremely visible (usually have a width of 0,5 mm). Furthermore, metals and pastes used in the realisation of such electrical contacts are susceptible to corrosion by the electrolyte, namely in particular by the redox couple iodine iodide (the most performant mediator and used widely).

In this context it is included the solution according to the present invention, which aims to solve the problem of performance degradation of the electrical contact as the temperature increases, as well as to increase the transparency, realising a contact not purely electric but more specifically electrochemical. In the context of the present invention, by the term "electrochemical contact" is meant a contact obtained through an electrolytic solution with a redox shuttle.

Moreover, such interconnection, being made of the same materials of the DSC, as will be shown below, does not present the problems of interaction with the electrolyte which are found instead with the materials commonly used in the known art.

These and other results are obtained according to the present invention proposing to provide electrochemical vertical connections, ie consisting of an electrolytic solution with a redox shuttle, perfectly camouflaged within the module, being constituted by a structure very similar to that of the cell itself. The vertical electrochemical connections according to the present invention are resistant to thermal and mechanical stresses, highly transparent and chemically inert to the electrolyte.

The purpose of the present invention is therefore to provide a vertical electrochemical contact of photoelectrochemical cells that allows to overcome the limits of the solutions according to the prior art and to obtain the technical results previously described. Further object of the invention is that said vertical contact can be made with substantially limited costs, both as regards production costs and as regards management costs.

Another object of the invention is to provide a vertical electrochemical contact of photoelectrochemical cells which is substantially simple, safe and reliable.

It is therefore a specific object of the present invention a photovoltaic module comprising two overlapped panels, at least one of which being transparent or semitransparent, and each being constituted by a flat substrate covered, on the side facing towards the other panel, by an electrically conductive coating divided into a plurality of adjacent regions electrically insulated by means of a corresponding number of interruptions, among said panels a plurality of adjacent dye-sensitized solar cells being interposed, one per each electrically insulated region, each of the dye-sensitized solar cells being comprised of a photo- electrode, positioned on the conductive coating of one of the two substrates, a counter-electrode, positioned on the conductive coating of the other substrate, and a liquid electrolyte interposed between said photo-electrode and said counter-electrode, wherein said adjacent dye- sensitized solar cells are connected in series by means of a corresponding plurality of vertical connections, connecting an electrically insulated region of the electrically conductive coating of a substrate, in connection with a dye-sensitized solar cell, with the electrically insulated region of the electrically conductive coating of the opposite substrate, in connection with an adjacent dye-sensitized solar cell, wherein said vertical connections are electrochemical connections, i.e. are constituted by an electrolyte solution with a redox shuttle.

Preferably, according to the invention, said vertical connections comprise two opposed catalytic layers, respectively one catalytic layer per each panel, arranged in contact with the respective conductive coating, a dye support layer, in contact with said catalytic layer of the panel on which the photo-electrodes are located of the dye-sensitized solar cells of the photovoltaic module, said support layer being constituted by a material having the same chemical characteristics (redox potential) and physical characteristics (coefficient of thermal expansion) of the material of the photo-electrode and said dye having chemical characteristics (redox potential) that are the same as those of the dye of said photo-electrodes and a liquid electrolyte, interposed between said dye support layer and the catalytic layer of the opposite panel.

More preferably, according to the invention, said dye support layer is made with the same material as the photo-electrode of the cells of the module and moreover said dye of said dye support layer is the same as the photo-electrode of the cells of the module.

Still according to the present invention, said liquid electrolyte has chemical characteristics (redox potential) that are the sane as those of the cells of the module and preferably is the same as the cells of the module.

Preferably, according to the invention, said liquid electrolyte has a higher ionic concentration with respect to that of the cells of the module.

Last, always according to the invention, said photoelectrochemical cells are arranged according to the shape of stripes, and preferably are large between 5 and 8mm.

It is evident the effectiveness of the vertical electrochemical contacts of photoelectrochemical cells according to the present invention, which, exploiting the characteristics of passive electrochemical devices, can be made of different colors and/or be highly transparent; are resistant with increasing temperature and at any contacts with the electrolyte of the DSC cells; can be integrated in the process of manufacture of DSC photovoltaic modules; offer a viable solution to realise photovoltaic modules (and therefore photovoltaic glazing), according to the Dye Solar Cell technology, with high characteristics of uniformity, the last feature being essential to meet the requirements of the BIPV (Building-integrated photovoltaics) solutions.

The invention will be hereinafter described for illustrative but not limitative purposes, with particular reference to some illustrative examples, with particular reference to the figures of the accompanying drawings, wherein: - Figure 1 shows schematically the configuration of a connection of type Z between two cells of a photoelectrochemical module, according to the prior art,

- Figure 2 shows schematically the configuration of a connection of type Z between two cells of a photoelectrochemical module, according to a first embodiment of the present invention,

- Figure 3A shows schematically a substrate for the photo-electrode of a photoelectrochemical module according to a second embodiment of the present invention, prior to mating with the corresponding substrate of the counter-electrode,

- Figure 3B shows schematically a substrate for the counter- electrode to the substrate corresponding to the photo-electrode of Figure 3A, prior to mating,

- Figure 4 shows a graph of the efficiency (expressed in terms of efficiency measured at a reference temperature of 25 °C) with changes in temperature (expressed in °C) of a photoelectrochemical module realised according to the present invention,

- Figure 5 shows two side by side photographs, respectively one to the left relative to a module wherein the vertical connections are made according to the prior art and one to the right relative to a module wherein the vertical connections are made according to the teachings of the present invention, and

- Figure 6 shows a graph of the efficiency (expressed as the percentage of incident light power converted into useful power) to vary the intensity of illumination (expressed in W/m ² ) of two different types of photoelectrochemical module realised second the present invention, respectively, with cells arranged into stripes of 5mm and 10mm.

As previously said, an objective of the present invention is the realisation of vertical electrochemical connections perfectly camouflaged inside of a photoelectrochemical module, as constituted by a structure very similar to that of the cells of the same photoelectrochemical module.

With reference to Figure 2, a vertical electrochemical contact according to the present invention is generally designated by the reference numeral 20 and is constituted by two layers of catalyst 21 (for example, platinum, PEDOT, gold, CoS), respectively arranged on the conductive coating 11 of one of the two substrates 10 and the conductive coating 11 of the other substrate 10; by one layer 22, disposed on the catalyst layer 21 which in turn is located on the conductive coating 11 of the substrate 10 on which is disposed the photo-electrode 13 of the cells of the photoelectrochemical module (but which could also be located on the conductive coating 11 of the opposite substrate, the choice of the substrate 10 on which the photo-electrode 13 is disposed of the cells of the photoelectrochemical module having aesthetic reasons but not reasons of operation), said layer 22 being formed with a material having chemical characteristics (redox potential) and physical characteristics (coefficient of thermal expansion) identical to that used for the realisation of the photo-electrode 13 of the cells of the photoelectrochemical module (preferably the same material, generally a porous titanium oxide), and supporting a dye that has chemical characteristics (redox potential) identical to that used for the realisation of the photo-electrode 13 of the cells of the photoelectrochemical module (preferably the same dye) and by a liquid electrolyte 23 interposed between said layer 22 and the catalyst layer 21 positioned on the conductive coating 11 of the other substrate 10 (which may be the same electrolyte of the electrochemical cells of the module or in any case an electrolyte which has the same chemical characteristics (redox potential), but which preferably has a greater ionic concentration).

Each vertical electrochemical contact 20 according to the Present

Invention is separated from the cells and the flank by means of the same encapsulant 16 which laterally delimits each cell.

It is readily apparent that the structure of the vertical electrochemical contact 20 according to the present invention is extremely similar to the structure of the photoelectrochemical cells of the same module, but has a purely resistive characteristic.

The connection path through the vertical electrochemical contact of the present invention can be modeled, taking into account the electrochemical nature of the contact, with two interface resistances (which decrease with increasing temperature) and the electrolyte ion diffusion resistance (which also decreases with increasing temperature). The mismatches of thermal and mechanical expansion, that typically characterise the connections made according to the solutions of the prior art, are overcome in this case by the fact that the materials used to produce respectively the cells and the vertical contact are the same. Furthermore, the contact of the two electrodes is guaranteed by the liquid electrolyte injected under vacuum.

The process for the production of the vertical electrochemical contact 20 according to the present invention provides that the same can be performed through various deposition techniques (screen printing, ink jet printing, dispensing, sputtering, CVD, spray), printing a catalyst (platinum, PEDOT, gold, CoS) on the conductive coating of the two substrates that have to be coupled. On one of the two substrates it is subsequently deposited a layer of material that has chemical characteristics (redox potential) and physical characteristics (coefficient of thermal expansion) identical to those of the photo-electrode of the cells that constitute the photoelectrochemical module, generally mesoporous titanium dioxide, according to the solutions of the prior art relating to the technology of photoelectrochemical cells. After calcination in an oven at 500 °C, or even at low temperatures (100 °C) with addition of pressure, the substrate on which the layer has been deposited that will form the photo-electrode is immersed in a solution of dye according to the prior art relating to the technology of photoelectrochemical cells. Subsequently, the substrate is washed and coupled to the other substrate through the intermediary of a sealant. In the interspace between the two substrates it is then added an electrolyte to connect the two surfaces, suitably chosen in relation to the catalyst used, always according to the teachings of the prior art.

It is self-evident that the realisation of the vertical electrochemical contact 20 according to the present invention is perfectly compatible with the process of realisation of a corresponding photoelectrochemical module, as will be better described below.

The device to be realised with this type of contacts, referred to as photoelectrochemical module, consists (as the corresponding devices obtainable according to the prior art) of two panels 24', 24", constituted by the substrates 10 and by the relative layers which cover them, which must be coupled together by means of the interposition of a sealant 16, between which an electrolyte 15 is inserted, containing an appropriate redox couple, responsible for the transport of charge between the two inner panels 24', 24".

The two panels 24', 24" can be distinguished in a panel of the photo-electrode 24' and a panel of the counter-electrode 24", respectively their name being due to the fact of being covered with the material that will form the photo-electrode 13 or with the material that will form the counter- electrode 14.

The device to be realised is actually a module with cells integrated together with the connection, on the same substrate, connected in series. This means that the final product will be in fact the composition of multiple devices (cells) divided by a connecting element (vertical contact).

With reference to Figure 3A, the panel of the photo-electrode 24' is constituted by an alternation of bands of T1O2 (representing the individual photo-electrodes 13 of the cells) and of catalyst (representing one of the electrodes 21 of the vertical contact 20). The panel of the photo-electrode 24' is then realised by printing a paste of T1O2 on the substrate 0 and, after calcination of the latter, depositing and processing the catalyst in the spaces where the Ti0 ₂ is not present. Areas where Ti0 ₂ is printed are the active areas of the device and therefore tend to occupy a greater area on the module. The areas where the catalyst is deposited, on the contrary, constitute an electrode for the vertical contact.

With reference to Figure 3B, in order to realise the panel of the counter-electrode 24", instead, a layer 25 of catalyst is deposited over the whole substrate 10. Subsequently, only in the zones 26 that during the coupling will result corresponding to the areas of the photo-electrode with the catalyst, the titanium dioxide is deposited, which is then calcined. Consequently, the panel of the counter-electrode 24" is constituted by areas that function as counter-electrode for the cells that are in the DSC module and by areas (those on which ΤΪΟ2 is subsequently printed) that function as second electrode for the vertical connection.

Both panels 24', 24" are then immersed in the dye.

Once the panels 24', 24" are realised, they are coupled, through the intermediary of a sealant deposited in such a way as to isolate from each other the internal cells and the vertical connections, according to the processes of the prior art.

After the coupling, both in active cells and in cells that form the vertical connection a respective electrolyte is placed.

Since the areas of the contacts and those of the cells are different, inside the two elements will flow a different current density (mA/cm ² ).

In particular, the cells of the photoelectrochemical module, which constitute the active areas of the module, will have a current density lower than that of the contacts (passive areas of the module). This suggests that, although one can use the same electrolyte both for cells and for contacts, it is preferable that the ionic concentration of the electrolyte of the contacts is greater than that of the cells, since the first must support a greater current density.

A greater current density, in fact, at the same concentration of the redox couple, can lead, especially at high levels of solar radiation on the module (and therefore of current), to a limitation of current inside the contact of diffusive type and, thus, to malfunctioning of the module.

It is also preferable to use for the electrolyte solvents that favor the ionic transport (ie slightly viscous) and the catalysis of the species.

To evaluate the characteristics of vertical electrochemical contacts according to the present invention and of photovoltaic modules of DSSC cells made integrating said contacts, and in particular to assess the impact of the geometric characteristics of the cells obtained according to the invention on the performance of the modules different types of modules with stripe like cells were realised, according to the process described above. Bearing in mind that the key geometric parameter of modules with stripe like cells is the width of the cells, two types of modules were realised, with stripes (cells) respectively having a width of 5mm and 10mm, while the width of the stripe like electrochemical contact in all cases was set equal to 5mm.

The modules realised for this purpose were obtained using materials readily available on the market, ie no study has been done of optimization of the materials. Consequently, the results obtained are not representative of the results achievable by the solution of the present invention, but are deemed by the applicant equally satisfactory, since they allow to demonstrate the possibility of a convenient use of the solution proposed according to the present invention.

It is also clear that, since in these examples materials have not been optimized for the connection (in particular the electrolyte), results have been obtained below the results obtainable according to the prior art. These experimental findings demonstrate, however, the correct functioning of the devices manufactured according to the present invention.

In particular, for the Ti0 ₂ was used the 18-nrt printable paste made by Dyesol. The dye used is a commercial ruthenium sensitizer, known as N-719 made by Dyesol. As electrolyte was used a commercial electrolyte with solvent based on acetonitrile (HPE-High Performance Electrolyte, Dyesol). The material for the catalyst is a commercial printable paste made by HCP Dyers srl. As the encapsulant Bynel by Dupont was used and the glass was Pilkington's TEC 8.

Figure 4 shows the trend of efficiency with the changing of temperature of a photoelectrochemical module realised according to the present invention and is equally representative of the trend of all modules produced in order to evaluate the applicability of the present invention. In particular, on the ordinate of the diagram the values of efficiency are shown expressed in relation to the efficiency measured at 25 °C, taken as a reference value. The trend shown in the diagram allows to see an increase in efficiency of 14% as the temperature varies from 25 °C to 60°C. The trend shown is mainly due to the decrease of the resistance of vertical connections realised according to the present invention compared to the contacts in accordance with the prior art.

Figure 5 shows the visual impact of the contacts according to the present invention in comparison with those made according to the prior art.

Finally, with reference to Figure 6, it is shown the measured efficiency of two different modules with electrochemical connection realised according to the present invention, under variation of the level of illumination. The tests carried out on the two photoelectrochemical modules, respectively a first module with 5mm large cell (width comparable with the width of the vertical electrochemical contact) and a second module with 0mm large cell (about twice the width of the vertical contact), have allowed to verify that the narrower cells provide higher efficiencies, since a lower amount of current flows within their contacts. In the case of 5mm wide cell, in fact, there are no problems due to diffusion limitation, being the cell area equal to the contact area. In the case of 10mm wide cell, instead, further limitations are observed due mainly to diffusion phenomena.

For both geometries, however, further optimization of materials is required.

Taking into account that the preferable intervals of width are determined in a first approximation by the sheet resistance of substrates, by the levels of current that the cell provides and by the inactive spaces of the module, and the amount of ions at the disposal of the electrolyte used for the connection, the preferred cell interval width can be from 5 to 8mm, while, according to the thin-film technologies of the prior art, a preferred width of cell varies from 6/7mm up to 12mm, and still more preferably has a value of around 10mm .

The present invention has been described for illustrative but not limitative purposes, according to its preferred embodiments, but it is to be understood that variations and/or modifications may be made by those skilled in the art without departing from the relevant scope of protection, as defined by the claims attached.