WO2013182955A2 - Back-sheet for photovoltaic modules comprising back-contact solar cells - Google Patents

Back-sheet for photovoltaic modules comprising back-contact solar cells Download PDF

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Publication number
WO2013182955A2
WO2013182955A2 PCT/IB2013/054434 IB2013054434W WO2013182955A2 WO 2013182955 A2 WO2013182955 A2 WO 2013182955A2 IB 2013054434 W IB2013054434 W IB 2013054434W WO 2013182955 A2 WO2013182955 A2 WO 2013182955A2
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WO
WIPO (PCT)
Prior art keywords
layer
conductive material
electrically conductive
substrate
sheet
Prior art date
Application number
PCT/IB2013/054434
Other languages
French (fr)
Other versions
WO2013182955A3 (en
Inventor
Elisa BACCINI
Luigi MARRAS
Bruno Bucci
Original Assignee
Ebfoil S.R.L.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from IT000132A external-priority patent/ITVI20120132A1/en
Priority claimed from IT000133A external-priority patent/ITVI20120133A1/en
Priority claimed from IT000266A external-priority patent/ITVI20120266A1/en
Application filed by Ebfoil S.R.L. filed Critical Ebfoil S.R.L.
Publication of WO2013182955A2 publication Critical patent/WO2013182955A2/en
Publication of WO2013182955A3 publication Critical patent/WO2013182955A3/en

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/042—PV modules or arrays of single PV cells
    • H01L31/048—Encapsulation of modules
    • H01L31/049—Protective back sheets
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/042—PV modules or arrays of single PV cells
    • H01L31/05—Electrical interconnection means between PV cells inside the PV module, e.g. series connection of PV cells
    • H01L31/0504—Electrical interconnection means between PV cells inside the PV module, e.g. series connection of PV cells specially adapted for series or parallel connection of solar cells in a module
    • H01L31/0516—Electrical interconnection means between PV cells inside the PV module, e.g. series connection of PV cells specially adapted for series or parallel connection of solar cells in a module specially adapted for interconnection of back-contact solar cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy

Definitions

  • the present invention relates to the field of photovoltaic modules.
  • the present invention relates to the field of novel back-contact back-sheets used in photovoltaic modules.
  • the present invention relates to a back-contact back-sheet to be applied to the surface of a back-contact back-sheet for a photovoltaic module wherein a plurality of back- contact solar cells is enclosed.
  • Solar cells are used for converting solar light into electrical energy by means of the photovoltaic effect. Solar cells are, thus, one of the most promising alternative energy sources for replacing fossil fuels. Solar cells are formed by semiconductor materials and assembled so as to form so called photovoltaic modules, which are in turn grouped in order to form photovoltaic plants to be typically installed on building roofs or the like.
  • groups of solar cells grouped in series through appropriate electrical conductors called “ribbons" are typically encapsulated by means of an encapsulating material such as for example a copolymer of ethylene and vinyl acetate, commonly known as EVA.
  • EVA ethylene and vinyl acetate
  • the encapsulating material enclosing the solar cells is then inserted between a surface layer and a base layer or back-sheet, so as to complete the photovoltaic module.
  • the surface layer, or main surface of the module covers the surface of the module exposed to the sun and allows the solar light to reach the cells.
  • the back-sheet carries out a multiplicity of tasks. It guarantees protection of the encapsulating material and of the solar cells from environmental agents, while simultaneously preventing the electrical connections from oxidizing. In particular, the back-sheet prevents moisture, oxygen and other factors depending on the atmospheric conditions from damaging the encapsulating material, the cells and the electrical connections.
  • the back-sheet also provides for electrical insulation for the cells and the corresponding electrical circuits. Furthermore, the back-sheet has to have a high degree of opacity due to esthetic reasons and high reflectivity in the part oriented toward the sun for functional reasons.
  • the electrical connection in photovoltaic modules comprising traditional solar cells occurs on both the front and the rear side of the cell, thereby causing shading problems.
  • the front electrode i.e. the electrode exposed to light radiation
  • H-patterning a technique called "H-patterning" which causes shading of the surface exposed to light radiation, due to the presence of metallic traces screening the light incident upon the front surface of the cell.
  • traditional electric contacts cause the efficiency of solar cells and modules to decrease.
  • Back-contact cells are a new, more efficient and cost-effective generation of photovoltaic cells, wherein the contacts with both electrodes of the cell are transferred to the rear side of the cell, i.e. onto the side not exposed to the light radiation.
  • MWP Metallization Wrap Through
  • Back-contact cells pose new technological problems concerning the design and the structure of the modules adapted to accommodate them.
  • the back-sheet has to be designed so as to support the connecting circuit onto which connections to both electrodes (base and emitter) of the cell are brought.
  • One of the solutions to this problem is the so called back-contact back-sheet, which is an evolution of the traditional back-sheet in which the connecting circuit is implemented directly on the surface of the back-sheet facing the cell.
  • Back-contact cell 600 is arranged between an upper encapsulating material layer 450 and a lower encapsulating material layer 400.
  • Cell 600 and encapsulating material layers 400 and 450 are then enclosed between a surface layer 800 typically made of glass or of a transparent and anti reflective material and the back-sheet 200, which may be a back-contact back-sheet.
  • FIG 1 there are also visible the paths of electrically conductive material making up the connecting circuit 220c to the electrodes of the solar cell.
  • back-sheet 200 is a back-contact back-sheet
  • the connecting circuit 220c is formed directly on the surface of the lower-lying insulating substrate and firmly fixed thereto.
  • the connecting circuit 220c is used so as to ensure an electric contact with both electrodes (i.e. with the base and the emitter) of solar cell 600.
  • the paths of electrically conductive material are provided with pads 222, marking the points of the connecting circuit which are to be electrically connected with a contact to one of the electrodes formed on the surface of cell 600.
  • the procedure of assembling a photovoltaic module such as that shown in Figure 1 is typically performed in the manner described in the following.
  • the lower encapsulating material layer 400 to be arranged between the cell 600 and the back- contact or back-contact back-sheet 200 is pierced so that, after the module has been completed, the holes formed in the lower encapsulating layer 400 correspond to areas where pads 222 for contact with the electrodes are arranged.
  • the pierced encapsulating material layer 400 is then laid on top of a back-sheet or back-contact back-sheet 200, so that the holes of the lower encapsulating material layer 400 correspond or are aligned to pads 222, in such a way as to leave pads 222 exposed toward the outside.
  • a lump or drop of an electrically conductive material is then deposited onto pads 222 of the conductive paths of the connecting circuit formed on the surface of the back-sheet or of the back-contact back-sheet 200.
  • the surface of pads 222 is left exposed by the holes of the lower encapsulating material layer 400.
  • the conductive material deposited onto pads 222 may for example comprise a conductive paste of the type knows as "Electro Conductive Adhesive" (ECA).
  • the cells 600 to be embedded in the module are placed onto the lower encapsulating material layer 400 so that each contact element with the electrodes formed on the rear surface of the cell comes into contact with the lump of conductive paste applied to one of the pads 222 and exposed to the contact with cell 600 through one of the holes of the lower encapsulating material layer 400.
  • the upper encapsulating material layer 450 is then placed onto the upper surface of the cell 600, opposite the rear surface in contact with the conductive paste applied to pads 222.
  • a layer 800 of a transparent and antireflective material is laid onto the upper encapsulating material layer 450.
  • this can be turned upside down and subsequently laminated in vacuum at a temperature between 145 °C and 165 °C for a time interval variable between 8 and 18 minutes.
  • Figure 2a shows the structure of the module before the lamination process.
  • the components of the module, stacked as previously described, are singularly distinguishable.
  • Figure 2a shows a stack comprising, starting from the bottom and moving toward the top of the figure, the back-sheet or back-contact back-sheet 200 with conductive pads 222 upon which conductive paste 300 has been applied, the lower encapsulating material layer 400, the cell 600, the upper encapsulating material 450 and surface layer 800.
  • Electrical connection to the electrodes (base and emitter) of cell 600 is provided for by contact points 620 and 640 arranged on the rear side of cell 600, i.e. on the side facing the connecting circuit 220c and the back-sheet 200.
  • Figure 2b schematically shows the structure of the module after the lamination process has taken place.
  • the structure is arranged into a vacuum chamber from which the air is evacuated by means of pumps.
  • a pressure is then applied to the structure so as to compact the layers of which the photovoltaic module structure is comprised while simultaneously maintaining a vacuum in the area in which the module is situated.
  • the whole cycle has preferably a total duration less than 18 minutes.
  • the cycle preferably occurs at a temperature between 140 °C and 165 °C.
  • the lamination results in the hardening of the conductive paste 300 through its polymerization, thus causing cells 600 to attach to back-sheet 200. Furthermore, a task of the lamination process is also causing melting and subsequent polymerization of the upper and lower encapsulating material layers 450 and 400. In this manner, the encapsulating material of the lower layer 400, by melting, fills all void spaces between the conductive paste 300, the back- sheet or back-contact back-sheet 200 and the rear surface of cells 600. Furthermore, after polymerizing, the upper encapsulating material layer 450 exerts also an adhesive action between the surface layer 800 and the outer surface of cell 600 in contact with upper encapsulating material 450. Analogously, the lower encapsulating material layer 400, after polymerizing, exerts also an adhesive action between the rear surface of cells 600 and back- sheet 200.
  • the back-contact back-sheet mentioned above has been developed, wherein the connecting circuit is firmly fixed to the surface of a substrate facing the inside of the photovoltaic module.
  • the connecting circuit has been implemented on the basis of a copper foil, due to the excellent electrical conduction properties of this material.
  • it has also been proposed to depose a thin film of a metal which is less prone to oxidation and more resilient than copper, for example nickel, onto the surface of the copper foil within which the connecting circuit is to be obtained.
  • copper turns out to be rather costly.
  • an object of the present invention is to provide a back-contact back-sheet for photovoltaic modules and a method of production thereof, that allow these problems to be overcome.
  • one of the objects of the present invention is to provide a back-contact back-sheet and a method for producing such a back-contact back-sheet, wherein the connecting circuit is comprised of a relatively cost-effective material, while nevertheless guaranteeing a high electrical conductivity and an effective electrical contact with the contact points formed on the electrodes of the photovoltaic cells.
  • a further object of the present invention is to provide a back-contact back-sheet and a method for implementing such back-contact back-sheet, wherein the surface of the connecting circuit is free from any insulating oxide layer.
  • a back-contact back-sheet for photovoltaic modules and a method of production of such back-contact back-sheet are provided based on the new and inventive concept that the connecting circuit, traditionally implemented in copper, may be comprised of aluminium.
  • the present invention is further based on the inventive concept that the surface of the connecting circuit to the electrodes of the solar cells may be formed by means of a thin film of a metal such as silver.
  • the superficial metal film may be formed by means of a deposition process, preferably performed in a vacuum chamber.
  • the present invention is based on the new and inventive concept according to which the aluminium foil of which the connecting circuit is comprised may undergo a surface treatment aimed at removing the oxide layer from its surface, before the deposition of the protective metal layer onto the surface of the aluminium foil has been performed, so as to improve the electrical conductivity between the cells and the circuit.
  • the present invention also provides a method for producing a back contact back sheet for a photovoltaic module as defined in claim 8 and in the following description.
  • Figure 1 shows an exploded view of a portion of a photovoltaic module comprising back-contact cells
  • Figure 2a shows a cross-sectional view of the structure of a photovoltaic module of the type shown in Figure 1 before the lamination process;
  • Figure 2b shows a cross-sectional view of the structure of a photovoltaic module of the type shown in Fig. 1 after the lamination process;
  • Figure 3 shows a cross-sectional view of a back-contact back-sheet.
  • Fig. 3 schematically shows a back-contact back-sheet for a photovoltaic module commonly used in the state of the art.
  • the air-side of the photovoltaic module is arranged at the bottom of Fig. 3, below the back-contact back-sheet 200.
  • Back-contact back-sheet 200 comprises an insulating substrate or complex 210 exposed to the air-side of the photovoltaic module.
  • the insulating substrate 210 comprises a first insulating layer 212, an intermediate layer 214 and a second insulating layer 216.
  • First insulating layer 212 has a surface exposed to the air-side of the photovoltaic material and is used as a barrier against moisture, UV rays, oxygen and other external agents which might penetrate into the photovoltaic module and cause damages to some constituent parts thereof or cause degradation of the polyurethane-like or polyester-like adhesive which might turn yellow.
  • the first insulating layer 212 may comprise a polymer such as polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), or other polymers.
  • the first insulating layer 212 may have a thickness in the range of 25 to 75 ⁇ or greater.
  • Intermediate layer 214 is typically comprised of a material non transparent to water vapour, e.g. aluminium. Intermediate layer 214 preferably has a thickness between 8 and 25 ⁇
  • Second insulating layer 216 typically comprises a polymer such as, for example, polyethylene terephthalate (PET), polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), polyethylene naphthalate (PEN), or the like. Second insulating layer 216 may have a thickness in the range of 125 to 350 ⁇ or greater.
  • insulating substrate 210 comprises only a first insulating layer 212 and a second insulating layer 216, directly applied onto the inner surface of first insulating layer 212, without the presence of any intermediate layer 214. It is also possible to implement back-contact back-sheets wherein the insulating substrate 210 is exclusively comprised of one layer comprising, for example, one or more polymers such as PET, PVF, PVDF, PEN and the like.
  • An electrically conductive material layer 220 is then applied to the inner surface of second insulating layer 216, opposite the surface of the second insulating layer 216 facing the intermediate layer 214. Electrically conductive material layer 220 is applied so as to be firmly fixed to second insulating layer 216. Electrically conductive material layer 220 may have a thickness approximately included in the range of 25 to 70 pm. In the back-contact back-sheets known from prior art, the material chosen for electrically conductive material layer 220 is typically copper.
  • Electrically conductive material layer 220 is usually applied onto insulating substrate 210 as a continuous foil. After being applied to the inner surface of insulating substrate 210, electrically conductive material layer 220 is processed so as to form a pattern comprising elongated electrically conductive elements such as paths, tracks, etc. Such a pattern forms the connecting circuit 220c to the electrodes of the solar cells.
  • the connecting circuit 220c is not formed as a continuous layer on the inner surface of the second insulating layer 216. Hence, the connecting circuit usually leaves exposed portions of the inner surface of the second insulating layer 216 onto which it is applied.
  • connecting circuit 220c may be formed in the electrically conductive material layer 220 by means of one of the techniques commonly used for producing printed circuit boards.
  • connecting circuit 220c may be formed in conductive layer 220 by means of optical lithography, wherein a photo resist layer is uniformly applied to the surface of conductive layer 220, exposed according to the pattern to be reproduced and subsequently developed. The surface comprising the developed photo resist then undergoes a chemical etch, which produces the desired pattern. Finally, the residual photo resist is removed.
  • connecting circuit 220c may be formed within conductive material layer 220 by means of an ablation process, to be achieved by mechanical means such as a milling machine or by evaporation using for example a laser.
  • conductive layer 220 may be provided as a foil previously printed in the form of a connecting circuit 220c. The pre-printed foil 220 is then applied to insulating substrate 210.
  • Conductive layer 220 further comprises pads 222 formed in predetermined positions of the pattern of which the connecting circuit 220c in electrically conductive layer 220 is comprised.
  • Pads 222 are adapted to be brought into electrical contact with the ohmic contacts formed on the surface of the electrodes of the solar cells by means of a lump of conductive material.
  • the ohmic contacts may, for example, be those indicated by reference numeral 620 and 640 in Figures 2a and 2b. Pads 222 thus ensure the electrical contact with both electrodes (base and emitter) of the solar cells mounted in the photovoltaic module.
  • the back-contact back-sheet 200 shown in Figure 3 further comprises a dielectric material layer 240 deposited onto the inner side of back-contact back-sheet 200, i.e. onto the side facing the cells and opposite the air-side.
  • This dielectric material layer 240 is optional and may not be present in other embodiments of the back-contact back-sheet according to the invention.
  • Dielectric material layer 240 is usually formed by means of a screen printing process, so that the dielectric material entirely covers the portions of the surface of second insulating layer 216 left exposed by conductive layer 220 applied thereupon. Dielectric material layer 240 thus accomplishes the task of electrically insulating two adjacent elements or paths of the pattern formed by connecting circuit 220c which are electrically disconnected with each other. Furthermore, dielectric material layer 240 also performs the task of hindering or neutralizing surface currents which might possibly flow on the surface of substrate 210 upon which electrically conductive material layer 220 and a portion of dielectric material layer 240 are fixed. Dielectric material layer 246 is deposited so as to leave conductive layer 220 partially exposed.
  • dielectric material layer 246 is provided with openings 242 situated in correspondence to pads 222 of connecting circuit 220c and used for contacting the electrodes of the photovoltaic cells.
  • dielectric layer 240 leaves conductive pads 222 exposed toward the inner side of back-contact back-sheet 200, i.e. toward the side facing the cell.
  • protective layer 260 may comprise an organic material, which is generally deposited through screen printing.
  • the exposed portions of conductive layer 220 may be protected by a metallic film originally applied to the exposed surface.
  • electrically conductive material layer 220 comprises copper, it can be protected by means of a thin nickel film deposited onto the surface of the conductive material layer 220 comprising copper.
  • the electrically conductive material layer and, hence, the connecting circuit have been usually formed from copper, on whose surface has been optionally applied a thin film of, e.g. nickel, tin, or other metals or of a protective organic compound (OSP Organic Surface Protection).
  • OSP Organic Surface Protection e.g. nickel, tin, or other metals or of a protective organic compound (OSP Organic Surface Protection).
  • copper is a material which, in spite of providing high electrical conductivity, entails considerable costs.
  • the present invention proposes to replace copper with aluminium as the basic material for implementing the connecting circuit 220c included in a back-contact back-sheet of one of the types previously described with reference to Fig. 3.
  • aluminium as a main component of the connecting circuit to the electrodes of the photovoltaic cells is limited by the drawbacks and the problems related to this material. Indeed, aluminium does have good properties of electrical conductivity. However, aluminium also entail the drawback of an oxide layer forming immediately after the material surface is exposed to the air. Aluminium oxide, or alumina, is notoriously a strong electrical insulator.
  • the connecting circuit is formed in such a manner that pads 222 formed on its surface are lead into electrical connection with contact points 620 and 640 on the electrodes of the cells shown, for example, in Figs. 2a and 2b. Therefore, at least the surface portion of the connecting circuit 220c occupied by pads 222 must be free of any superficial insulating film which may reduce the electrical contact between the connecting circuit 220c and the contact points 620 and 640 on the electrodes of the solar cells. As a consequence, aluminium has never been considered as a possible conductive material for layer 220 in which the connecting circuit 220c shown in Fig. 1 is formed.
  • the present invention provides a back-contact back-sheet wherein the connecting circuit 220c is implemented on the basis of an aluminium foil. Furthermore, the present invention provides a method for effectively producing such a back-contact back-sheet, wherein the connecting circuit comprises aluminium.
  • the back-contact back-sheet according to an embodiment of the present invention has an analogous structure to that of back-contact back-sheet 200 previously described with reference to Fig. 3.
  • One of the differences between the back-contact back-sheet according to the present invention with respect to the back-contact back-sheet of Fig. 3 is the fact that the electrically conductive material layer 220 fixed to the inner surface of insulating substrate 210 comprises aluminium.
  • conductive material layer 220 wherein conducting circuit 220c is obtained is covered by a thin film of a metal different from aluminium.
  • the protective thin film comprises silver.
  • the protective metal film has a thickness in the range of 12 to 200 nm and, preferably, of 40 to 80 nm.
  • the back- contact back-sheet comprises an insulating substrate 210 and an electrically conductive material layer 220 wherein the connecting circuit 220c is formed, without the dielectric material layer 240 shown in Fig. 3.
  • a substrate 210 is formed which, as previously described, may comprise one, two, or three layers. Each layer may comprise polymeric or metallic materials. Insulating substrate 210, thus implemented, has an outer surface exposed toward the air-side of the photovoltaic module and an inner surface opposite the outer surface and exposed toward the inside of the photovoltaic module.
  • electrically conducive material layer 220 is prepared, within which connecting circuits 220c is to be obtained.
  • Layer 220 is prepared by placing an aluminium foil of the desired thickness into a plating apparatus from which the air is evacuated by pumps, thereby creating a vacuum. More specifically the pressure within the plating apparatus is caused to drop down to a value of the order of 10-6 torr.
  • One surface of the aluminium foil is then treated with a plasma so as to remove by means of etching the native oxide layer originally present on the aluminium surface.
  • This surface of the aluminium foil or tape undergoing the surface treatment will be in the following referred to as the "first surface”.
  • the surface of the aluminium foil or tape opposite the first surface will instead be referred to as the "second surface”.
  • the plasma used for the treatment may comprise a hydrogen plasma or a plasma based on a different gas or gas mixture.
  • the plasma treatment is performed preferably within a vacuum chamber included in the plating apparatus. In this manner, the oxide on the first surface of the aluminium foil is prevented from reforming after being removed by means of the plasma treatment.
  • a protective thin film of a metal is deposited onto the previously treated surface.
  • the metal forming the protective thin film preferably comprises silver or a metal alloy comprising silver.
  • the deposition of the thin film is preferably performed by means of physical vapour deposition
  • the plating apparatus may comprise a crucible into which a predetermined quantity of the metal to be evaporated may be placed in solid form. Thereafter, the crucible within the plating apparatus is heated up, for example, by letting an electrical current flow from one end to the opposite end of the crucible.
  • the metal in the crucible initially melts and subsequently vaporizes or sublimes due to the low pressure within the plating apparatus.
  • the particles of the metal to be deposited move, e.g. upwards, until they impinge onto the aluminium foil surface facing the crucible.
  • This surface kept at an appropriately low temperature, receives the evaporated material in the form of condensed metal.
  • the surface of the aluminium foil facing the crucible is the first surface previously treated with a plasma, i.e. the surface from which the oxide film has been removed.
  • the vapours of the metal to be deposited condense thereon.
  • the aluminium foil may be kept at an appropriate temperature, for example at room temperature, so that the condensation of the vapours of the metal to be deposited occurs more promptly.
  • the metal to be deposited initially in the form of a vapour, is thus deposited so as to remain steadily fixed to the aluminium foil, after the metal vapours have impinged onto the foil surface and have condensed. In this manner, a protective film can be formed onto the first surface of the aluminium foil from which the oxide film has been previously removed.
  • Two or more crucibles may be also used instead of only one, so as to enable simultaneous deposition of more materials and to form a protective film comprising different chemical species, for example a metal alloy, onto the first surface of the aluminium foil.
  • a metal alloy for example, together with or as an alternative to silver, other metals may be deposited such as nickel, tin, cadmium, tantalum, cobalt, or even copper itself, or else an aluminium-copper alloy.
  • silver is the most advantageous choice due to several reasons.
  • silver is the metal with the greatest electrical conductivity within the list of metallic materials provided above.
  • silver, being a noble metal is particularly stable in air, since a surface thereof exposed to the air hardly reacts with gases (oxygen, water vapour, nitrogen, carbon dioxide, etc.) normally present in air at standard pressures.
  • silver is compatible with silver-based adhesives (e.g. ECA) typically used in the field.
  • a protective film may thus be deposited comprising more than one metal.
  • the present invention enables deposition of multiple protective films comprised of different metals or metal alloys and super imposed to each other.
  • the second surface of the foil is preferably free and exposed toward the outside of the foil.
  • the second surface of the aluminium foil is neither in contact with nor adheres to other surfaces of other elements. Therefore, the aluminium foil is singularly treated during the sequence of plasma etching and deposition of the thin metal film. So, the aluminium foil is neither fixed nor coupled to any substrate or support while the oxide film is removed and a thin metal film is deposited onto the first surface.
  • the aluminium foil does not require any support or substrate during the sequence of plasma etching and deposition onto the first surface. Furthermore, since the second surface is exposed toward the outside, it may also, where required or needed, be treated or modified, for example by means of any one of the well known techniques of surface treatments or by means of a deposition of a metal film, a dielectric film, a polymeric film, etc.
  • the sequence of etching and deposition onto the first aluminium surface preferably occurs within a process of the roll-to-roll type.
  • an aluminium tape is inserted into the plating apparatus wound so as to form a reel.
  • the aluminium tape has a desired thickness and width.
  • the width of the tape might be of about 1 m.
  • the aluminium tape After evacuating air and establishing a vacuum within the plating apparatus as described above, the aluminium tape unwinds from the starting reel or first reel along a predetermined path within the plating apparatus. At the end of this path, aluminium winds around an arrival reel or second reel, which is, along with the first reel, arranged within the plating apparatus. At a given point of the path between the first and the second reel, the plasma treatment is applied to the first surface of the aluminium tape unwound from the first reel, in order to remove the native oxide film. Subsequently, a thin metal film, preferably silver, is deposited onto the aluminium tape first surface previously treated. Deposition is performed at a point of the aluminium tape unwinding path arranged downstream from the point at which the plasma treatment is applied.
  • the expression "downstream” and "upstream” are understood to be referred to the unwinding direction of the tape, i.e. the direction going from the first to the second reel along the unwinding path of the aluminium tape.
  • the aluminium tape is caused to wind in the second reel. After the entire tape is wound in the second reel, the plating apparatus may be opened and the reel may be extracted, together with the aluminium tape whose first surface is covered by the protective metal film.
  • the process of preparing conductive layer 220 may thus be speeded up and optimized by operating not onto a single aluminium foil but, rather, onto the whole tape, which is to be cut so as to form the foils to be used in the respective photovoltaic modules.
  • aluminium foils may thus be obtained having a surface protected by a metal film preferably comprising silver.
  • the aluminium foil obtained by the process does not have any insulating oxide layer between the silver surface and the protective metal layer deposited thereon.
  • the second surface of the aluminium tape is free or exposed toward the outside during the sequence of plasma etching and deposition onto the tape first surface.
  • the two layers are coupled so as to be firmly fixed to each other.
  • an adhesive or a thermo-adhesive can be used, interposed between the surface of insulating substrate 210 facing the inside of the photovoltaic material and the surface of the conductive material layer 220 facing substrate 210.
  • Coupling occurs so that the second surface of conductive layer 220 opposite the treated and plated surface faces the inner surface of insulating substrate 210, i.e. the surface of insulating substrate 210 opposite the surface facing the air-side. In this manner the first surface of the aluminium foil, plated with a thin metal film, remains exposed toward the outside.
  • conductive material layer 220 directly adheres to the inner surface of substrate
  • first surface directly adhering to the second surface it is here intended that the first surface is firmly fixed to the second surface, while no further element or layer is present between the first and the second surface except, at most, an adhesive material, a thermo- adhesive material or in general an element enabling adhesion between the first and second surface.
  • the conductive material layer 220 is firmly fixed to the inner surface of substrate 210 of the photovoltaic module and, between conductive material layer 220 and the inner surface of substrate 210, no other element or layer is present except, where needed, an adhesive material, a thermo-adhesive material, or the like.
  • substrate 210 is produced by means of a process of the roll-to-roll type analogous to that described as a possible option for preparing the conductive material layer 220.
  • Coupling between substrate 210 and conductive material layer 220 is particularly advantageous when performed by means of a roll- to-roll process.
  • foils of a desired dimension for example 1 .7 m x 1 m
  • the conductive material layer is usually continuous and does not have any holes, channels or indentations.
  • electrically conductive layer 220 typically does not yet form a circuit.
  • the connecting circuit 220c must thus be formed within electrically conductive material layer 220.
  • Connecting circuit 220c may be formed by means of one of the techniques described above, for example optical lithography, mechanical milling, laser ablation, laser contouring, etc. Forming the connecting circuits 220c thus completes the process of producing a back-contact back-sheet according to the present invention.
  • a dielectric material film 240 may be deposited onto the surface of back-contact back-sheet 200 facing the inside of the photovoltaic module after forming the circuit.
  • the dielectric film 240 is thus deposited onto the plated surface of connecting circuit 220c formed in the conductive material layer 220 and onto the surface portion of substrate 210 upon which layer 220 is fixed and left exposed by connecting circuit 220c.
  • the reader is referred to the description provided above with reference to Fig. 3 for the feature and functionality of dielectric film 240 and for some examples of deposition methods.
  • a back-contact back-sheet has thus been formed wherein connecting circuit 220c is formed from aluminium instead of more costly materials such as, for example, copper.
  • the performances of the connecting circuit according to the present invention are guaranteed by a surface treatment comprising a sequence of a plasma etch, followed by the deposition of a thin film of a material not subject to oxidation and resilient such as, for example, silver.
  • a surface treatment comprising a sequence of a plasma etch, followed by the deposition of a thin film of a material not subject to oxidation and resilient such as, for example, silver.

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Abstract

The present invention proposes a back-contact back-sheet for a photovoltaic module comprising back-contact solar cells. The back-contact back-sheet comprises a substrate (210) having an outer surface exposed toward the air-side of the photovoltaic module and an inner surface opposite said outer surface and facing the inside of the photovoltaic module. The back-contact back-sheet further comprises a layer (220) of an electrically conductive material comprising aluminium adapted to be formed as a connecting circuit (220c) to the electrodes of said solar cells and directly adherent to the inner surface of the substrate (210). The layer of conductive material (220) comprises a protective metal film formed on the surface of the layer of conductive material (220) opposite the surface facing said substrate (210).

Description

BACK-SHEET FOR PHOTOVOLTAIC MODULES COMPRISING BACK-CONTACT SOLAR CELLS
TECHNICAL FIELD OF THE INVENTION
The present invention relates to the field of photovoltaic modules. In particular, the present invention relates to the field of novel back-contact back-sheets used in photovoltaic modules. Yet more in detail, the present invention relates to a back-contact back-sheet to be applied to the surface of a back-contact back-sheet for a photovoltaic module wherein a plurality of back- contact solar cells is enclosed.
STATE OF THE ART
Solar cells are used for converting solar light into electrical energy by means of the photovoltaic effect. Solar cells are, thus, one of the most promising alternative energy sources for replacing fossil fuels. Solar cells are formed by semiconductor materials and assembled so as to form so called photovoltaic modules, which are in turn grouped in order to form photovoltaic plants to be typically installed on building roofs or the like.
In order to form a photovoltaic module, groups of solar cells, grouped in series through appropriate electrical conductors called "ribbons", are typically encapsulated by means of an encapsulating material such as for example a copolymer of ethylene and vinyl acetate, commonly known as EVA. The encapsulating material enclosing the solar cells is then inserted between a surface layer and a base layer or back-sheet, so as to complete the photovoltaic module.
The surface layer, or main surface of the module, typically made of glass, covers the surface of the module exposed to the sun and allows the solar light to reach the cells. On the other hand, the back-sheet carries out a multiplicity of tasks. It guarantees protection of the encapsulating material and of the solar cells from environmental agents, while simultaneously preventing the electrical connections from oxidizing. In particular, the back-sheet prevents moisture, oxygen and other factors depending on the atmospheric conditions from damaging the encapsulating material, the cells and the electrical connections. The back-sheet also provides for electrical insulation for the cells and the corresponding electrical circuits. Furthermore, the back-sheet has to have a high degree of opacity due to esthetic reasons and high reflectivity in the part oriented toward the sun for functional reasons.
The electrical connection in photovoltaic modules comprising traditional solar cells occurs on both the front and the rear side of the cell, thereby causing shading problems. In particular, the front electrode, i.e. the electrode exposed to light radiation, is electrically contacted by means of a technique called "H-patterning" which causes shading of the surface exposed to light radiation, due to the presence of metallic traces screening the light incident upon the front surface of the cell. Hence, traditional electric contacts cause the efficiency of solar cells and modules to decrease.
Back-contact cells are a new, more efficient and cost-effective generation of photovoltaic cells, wherein the contacts with both electrodes of the cell are transferred to the rear side of the cell, i.e. onto the side not exposed to the light radiation.
Among the various families of back-contact cells, Metallization Wrap Through (MWP) cells turn out to be particularly efficient and easy to implement. In MWT cells, the contact to the front electrode is transferred to the rear side of the back cell by means of a through hole extending across the thickness of the semiconductor substrate.
Back-contact cells pose new technological problems concerning the design and the structure of the modules adapted to accommodate them. For example, the back-sheet has to be designed so as to support the connecting circuit onto which connections to both electrodes (base and emitter) of the cell are brought. One of the solutions to this problem is the so called back-contact back-sheet, which is an evolution of the traditional back-sheet in which the connecting circuit is implemented directly on the surface of the back-sheet facing the cell.
In Figure 1 the structure of a photovoltaic module comprising back-contact solar cells is shown. Back-contact cell 600 is arranged between an upper encapsulating material layer 450 and a lower encapsulating material layer 400. Cell 600 and encapsulating material layers 400 and 450 are then enclosed between a surface layer 800 typically made of glass or of a transparent and anti reflective material and the back-sheet 200, which may be a back-contact back-sheet.
In figure 1 there are also visible the paths of electrically conductive material making up the connecting circuit 220c to the electrodes of the solar cell. If back-sheet 200 is a back-contact back-sheet, the connecting circuit 220c is formed directly on the surface of the lower-lying insulating substrate and firmly fixed thereto. The connecting circuit 220c is used so as to ensure an electric contact with both electrodes (i.e. with the base and the emitter) of solar cell 600. In particular, the paths of electrically conductive material are provided with pads 222, marking the points of the connecting circuit which are to be electrically connected with a contact to one of the electrodes formed on the surface of cell 600. The procedure of assembling a photovoltaic module such as that shown in Figure 1 is typically performed in the manner described in the following.
The lower encapsulating material layer 400 to be arranged between the cell 600 and the back- contact or back-contact back-sheet 200 is pierced so that, after the module has been completed, the holes formed in the lower encapsulating layer 400 correspond to areas where pads 222 for contact with the electrodes are arranged. The pierced encapsulating material layer 400 is then laid on top of a back-sheet or back-contact back-sheet 200, so that the holes of the lower encapsulating material layer 400 correspond or are aligned to pads 222, in such a way as to leave pads 222 exposed toward the outside.
A lump or drop of an electrically conductive material is then deposited onto pads 222 of the conductive paths of the connecting circuit formed on the surface of the back-sheet or of the back-contact back-sheet 200. The surface of pads 222 is left exposed by the holes of the lower encapsulating material layer 400. The conductive material deposited onto pads 222 may for example comprise a conductive paste of the type knows as "Electro Conductive Adhesive" (ECA).
Subsequently, the cells 600 to be embedded in the module are placed onto the lower encapsulating material layer 400 so that each contact element with the electrodes formed on the rear surface of the cell comes into contact with the lump of conductive paste applied to one of the pads 222 and exposed to the contact with cell 600 through one of the holes of the lower encapsulating material layer 400. The upper encapsulating material layer 450 is then placed onto the upper surface of the cell 600, opposite the rear surface in contact with the conductive paste applied to pads 222. Finally, a layer 800 of a transparent and antireflective material is laid onto the upper encapsulating material layer 450.
After the structure has been prepared as described above, this can be turned upside down and subsequently laminated in vacuum at a temperature between 145 °C and 165 °C for a time interval variable between 8 and 18 minutes.
Figure 2a shows the structure of the module before the lamination process. The components of the module, stacked as previously described, are singularly distinguishable. In particular, Figure 2a shows a stack comprising, starting from the bottom and moving toward the top of the figure, the back-sheet or back-contact back-sheet 200 with conductive pads 222 upon which conductive paste 300 has been applied, the lower encapsulating material layer 400, the cell 600, the upper encapsulating material 450 and surface layer 800. Electrical connection to the electrodes (base and emitter) of cell 600 is provided for by contact points 620 and 640 arranged on the rear side of cell 600, i.e. on the side facing the connecting circuit 220c and the back-sheet 200. Figure 2b schematically shows the structure of the module after the lamination process has taken place. During the first lamination stage, the structure is arranged into a vacuum chamber from which the air is evacuated by means of pumps. A pressure is then applied to the structure so as to compact the layers of which the photovoltaic module structure is comprised while simultaneously maintaining a vacuum in the area in which the module is situated. The whole cycle has preferably a total duration less than 18 minutes. The cycle preferably occurs at a temperature between 140 °C and 165 °C.
The lamination results in the hardening of the conductive paste 300 through its polymerization, thus causing cells 600 to attach to back-sheet 200. Furthermore, a task of the lamination process is also causing melting and subsequent polymerization of the upper and lower encapsulating material layers 450 and 400. In this manner, the encapsulating material of the lower layer 400, by melting, fills all void spaces between the conductive paste 300, the back- sheet or back-contact back-sheet 200 and the rear surface of cells 600. Furthermore, after polymerizing, the upper encapsulating material layer 450 exerts also an adhesive action between the surface layer 800 and the outer surface of cell 600 in contact with upper encapsulating material 450. Analogously, the lower encapsulating material layer 400, after polymerizing, exerts also an adhesive action between the rear surface of cells 600 and back- sheet 200.
In order to simplify and optimize the assembly process of photovoltaic modules comprising back- contact cells, the back-contact back-sheet mentioned above has been developed, wherein the connecting circuit is firmly fixed to the surface of a substrate facing the inside of the photovoltaic module.
Traditionally the connecting circuit has been implemented on the basis of a copper foil, due to the excellent electrical conduction properties of this material. In order to prevent the surface layer of the connecting circuit from oxidizing or being damaged, it has also been proposed to depose a thin film of a metal which is less prone to oxidation and more resilient than copper, for example nickel, onto the surface of the copper foil within which the connecting circuit is to be obtained. However, using copper turns out to be rather costly.
OBJECT OF THE PRESENT INVENTION
In view of the problems mentioned above related to the back-contact back-sheets known from the prior art, an object of the present invention is to provide a back-contact back-sheet for photovoltaic modules and a method of production thereof, that allow these problems to be overcome.
In particular, one of the objects of the present invention is to provide a back-contact back-sheet and a method for producing such a back-contact back-sheet, wherein the connecting circuit is comprised of a relatively cost-effective material, while nevertheless guaranteeing a high electrical conductivity and an effective electrical contact with the contact points formed on the electrodes of the photovoltaic cells.
A further object of the present invention is to provide a back-contact back-sheet and a method for implementing such back-contact back-sheet, wherein the surface of the connecting circuit is free from any insulating oxide layer.
BRIEF DESCRIPTION OF THE INVENTION
According to the present invention, a back-contact back-sheet for photovoltaic modules and a method of production of such back-contact back-sheet are provided based on the new and inventive concept that the connecting circuit, traditionally implemented in copper, may be comprised of aluminium.
The present invention is further based on the inventive concept that the surface of the connecting circuit to the electrodes of the solar cells may be formed by means of a thin film of a metal such as silver. According to a further inventive concept on which the present invention is based, the superficial metal film may be formed by means of a deposition process, preferably performed in a vacuum chamber.
Furthermore, the present invention is based on the new and inventive concept according to which the aluminium foil of which the connecting circuit is comprised may undergo a surface treatment aimed at removing the oxide layer from its surface, before the deposition of the protective metal layer onto the surface of the aluminium foil has been performed, so as to improve the electrical conductivity between the cells and the circuit.
On the basis of these considerations, the back-contact back-sheet for photovoltaic modules according to independent claim 1 is proposed.
Further embodiments of the present invention will be provided by the dependent claims and the following description.
The present invention also provides a method for producing a back contact back sheet for a photovoltaic module as defined in claim 8 and in the following description. BRIEF DESCRIPTION OF THE FIGURES
Further features and advantages of the present invention will appear more clearly from the following description of the embodiments of the device and the method according to the present invention shown in the figures. In the figures identical and/or similar and/or corresponding parts are identified by the same reference numerals or letters.
In particular, in the figures:
Figure 1 shows an exploded view of a portion of a photovoltaic module comprising back-contact cells;
Figure 2a shows a cross-sectional view of the structure of a photovoltaic module of the type shown in Figure 1 before the lamination process;
Figure 2b shows a cross-sectional view of the structure of a photovoltaic module of the type shown in Fig. 1 after the lamination process;
Figure 3 shows a cross-sectional view of a back-contact back-sheet.
DETAILED DESCRIPTION
In the following, the present invention will be described with reference to particular embodiments as shown in the attached figures. However, the present invention is not restricted to the particular embodiments described in the following detailed description and shown in the figures. Rather, the described embodiments simply show several aspects of the present invention, whose scope is defined by the claims.
Further modifications and variations of the present invention will be clear for the skilled person. Consequently, the present invention should be considered as comprising all modifications and/or variations of the present invention, whose scope is defined by the claims.
Fig. 3 schematically shows a back-contact back-sheet for a photovoltaic module commonly used in the state of the art. The air-side of the photovoltaic module is arranged at the bottom of Fig. 3, below the back-contact back-sheet 200.
Back-contact back-sheet 200 comprises an insulating substrate or complex 210 exposed to the air-side of the photovoltaic module.
The insulating substrate 210 comprises a first insulating layer 212, an intermediate layer 214 and a second insulating layer 216. First insulating layer 212 has a surface exposed to the air-side of the photovoltaic material and is used as a barrier against moisture, UV rays, oxygen and other external agents which might penetrate into the photovoltaic module and cause damages to some constituent parts thereof or cause degradation of the polyurethane-like or polyester-like adhesive which might turn yellow. The first insulating layer 212 may comprise a polymer such as polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), or other polymers. The first insulating layer 212 may have a thickness in the range of 25 to 75 μιη or greater.
The inner surface of the first insulating layer 212, opposite the surface exposed to the air-side, faces the intermediate layer 214, which acts as a barrier against moisture and water vapour. Intermediate layer 214 is typically comprised of a material non transparent to water vapour, e.g. aluminium. Intermediate layer 214 preferably has a thickness between 8 and 25 μητ
The inner surface of intermediate layer 214, opposite the surface facing first insulating layer 212, then faces the second insulating layer 216, acting as an electrical insulator and a further barrier. Second insulating layer 216 typically comprises a polymer such as, for example, polyethylene terephthalate (PET), polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), polyethylene naphthalate (PEN), or the like. Second insulating layer 216 may have a thickness in the range of 125 to 350 μηι or greater.
In alternative embodiments of the back-contact back-sheet not shown in the figures, insulating substrate 210 comprises only a first insulating layer 212 and a second insulating layer 216, directly applied onto the inner surface of first insulating layer 212, without the presence of any intermediate layer 214. It is also possible to implement back-contact back-sheets wherein the insulating substrate 210 is exclusively comprised of one layer comprising, for example, one or more polymers such as PET, PVF, PVDF, PEN and the like.
An electrically conductive material layer 220 is then applied to the inner surface of second insulating layer 216, opposite the surface of the second insulating layer 216 facing the intermediate layer 214. Electrically conductive material layer 220 is applied so as to be firmly fixed to second insulating layer 216. Electrically conductive material layer 220 may have a thickness approximately included in the range of 25 to 70 pm. In the back-contact back-sheets known from prior art, the material chosen for electrically conductive material layer 220 is typically copper.
Electrically conductive material layer 220 is usually applied onto insulating substrate 210 as a continuous foil. After being applied to the inner surface of insulating substrate 210, electrically conductive material layer 220 is processed so as to form a pattern comprising elongated electrically conductive elements such as paths, tracks, etc. Such a pattern forms the connecting circuit 220c to the electrodes of the solar cells.
The connecting circuit 220c is not formed as a continuous layer on the inner surface of the second insulating layer 216. Hence, the connecting circuit usually leaves exposed portions of the inner surface of the second insulating layer 216 onto which it is applied.
After fixing conductive material layer 220 to the inner surface of insulating substrate 210, connecting circuit 220c may be formed in the electrically conductive material layer 220 by means of one of the techniques commonly used for producing printed circuit boards. For example, connecting circuit 220c may be formed in conductive layer 220 by means of optical lithography, wherein a photo resist layer is uniformly applied to the surface of conductive layer 220, exposed according to the pattern to be reproduced and subsequently developed. The surface comprising the developed photo resist then undergoes a chemical etch, which produces the desired pattern. Finally, the residual photo resist is removed.
Alternatively, connecting circuit 220c may be formed within conductive material layer 220 by means of an ablation process, to be achieved by mechanical means such as a milling machine or by evaporation using for example a laser.
As an alternative to the methods described above, conductive layer 220 may be provided as a foil previously printed in the form of a connecting circuit 220c. The pre-printed foil 220 is then applied to insulating substrate 210.
Conductive layer 220 further comprises pads 222 formed in predetermined positions of the pattern of which the connecting circuit 220c in electrically conductive layer 220 is comprised. Pads 222 are adapted to be brought into electrical contact with the ohmic contacts formed on the surface of the electrodes of the solar cells by means of a lump of conductive material. The ohmic contacts may, for example, be those indicated by reference numeral 620 and 640 in Figures 2a and 2b. Pads 222 thus ensure the electrical contact with both electrodes (base and emitter) of the solar cells mounted in the photovoltaic module.
The back-contact back-sheet 200 shown in Figure 3 further comprises a dielectric material layer 240 deposited onto the inner side of back-contact back-sheet 200, i.e. onto the side facing the cells and opposite the air-side. This dielectric material layer 240 is optional and may not be present in other embodiments of the back-contact back-sheet according to the invention.
Dielectric material layer 240 is usually formed by means of a screen printing process, so that the dielectric material entirely covers the portions of the surface of second insulating layer 216 left exposed by conductive layer 220 applied thereupon. Dielectric material layer 240 thus accomplishes the task of electrically insulating two adjacent elements or paths of the pattern formed by connecting circuit 220c which are electrically disconnected with each other. Furthermore, dielectric material layer 240 also performs the task of hindering or neutralizing surface currents which might possibly flow on the surface of substrate 210 upon which electrically conductive material layer 220 and a portion of dielectric material layer 240 are fixed. Dielectric material layer 246 is deposited so as to leave conductive layer 220 partially exposed. More specifically, dielectric material layer 246 is provided with openings 242 situated in correspondence to pads 222 of connecting circuit 220c and used for contacting the electrodes of the photovoltaic cells. Thus, dielectric layer 240 leaves conductive pads 222 exposed toward the inner side of back-contact back-sheet 200, i.e. toward the side facing the cell.
Finally, the portions of conductive layer 220 left exposed by dielectric material layer 246 are covered by a protective layer 260 in order to prevent the exposed surface of electrically conductive material 220 from being oxidised, corroded, scratched or damaged in general, thereby reducing its electrical conductivity and its adhesion to the conductive paste (e.g. ECA). Protective layer 260 may comprise an organic material, which is generally deposited through screen printing. Alternatively, the exposed portions of conductive layer 220 may be protected by a metallic film originally applied to the exposed surface. For example, if electrically conductive material layer 220 comprises copper, it can be protected by means of a thin nickel film deposited onto the surface of the conductive material layer 220 comprising copper.
In the photovoltaic modules produced so far, the electrically conductive material layer and, hence, the connecting circuit, have been usually formed from copper, on whose surface has been optionally applied a thin film of, e.g. nickel, tin, or other metals or of a protective organic compound (OSP Organic Surface Protection). However, copper is a material which, in spite of providing high electrical conductivity, entails considerable costs.
The present invention proposes to replace copper with aluminium as the basic material for implementing the connecting circuit 220c included in a back-contact back-sheet of one of the types previously described with reference to Fig. 3.
Using aluminium as a main component of the connecting circuit to the electrodes of the photovoltaic cells is limited by the drawbacks and the problems related to this material. Indeed, aluminium does have good properties of electrical conductivity. However, aluminium also entail the drawback of an oxide layer forming immediately after the material surface is exposed to the air. Aluminium oxide, or alumina, is notoriously a strong electrical insulator.
As described above, the connecting circuit is formed in such a manner that pads 222 formed on its surface are lead into electrical connection with contact points 620 and 640 on the electrodes of the cells shown, for example, in Figs. 2a and 2b. Therefore, at least the surface portion of the connecting circuit 220c occupied by pads 222 must be free of any superficial insulating film which may reduce the electrical contact between the connecting circuit 220c and the contact points 620 and 640 on the electrodes of the solar cells. As a consequence, aluminium has never been considered as a possible conductive material for layer 220 in which the connecting circuit 220c shown in Fig. 1 is formed.
The present invention provides a back-contact back-sheet wherein the connecting circuit 220c is implemented on the basis of an aluminium foil. Furthermore, the present invention provides a method for effectively producing such a back-contact back-sheet, wherein the connecting circuit comprises aluminium.
The back-contact back-sheet according to an embodiment of the present invention has an analogous structure to that of back-contact back-sheet 200 previously described with reference to Fig. 3. One of the differences between the back-contact back-sheet according to the present invention with respect to the back-contact back-sheet of Fig. 3 is the fact that the electrically conductive material layer 220 fixed to the inner surface of insulating substrate 210 comprises aluminium.
According to an embodiment of the back-contact back-sheet according to the present invention, conductive material layer 220 wherein conducting circuit 220c is obtained, is covered by a thin film of a metal different from aluminium.
According to an embodiment of the present invention, the protective thin film comprises silver. According to an embodiment of the present invention, the protective metal film has a thickness in the range of 12 to 200 nm and, preferably, of 40 to 80 nm.
According to a further embodiment of the present invention not shown in the figures, the back- contact back-sheet comprises an insulating substrate 210 and an electrically conductive material layer 220 wherein the connecting circuit 220c is formed, without the dielectric material layer 240 shown in Fig. 3.
In the following, a method of producing a back-contact back-sheet according to the present invention will be described.
At the beginning of the production process, a substrate 210 is formed which, as previously described, may comprise one, two, or three layers. Each layer may comprise polymeric or metallic materials. Insulating substrate 210, thus implemented, has an outer surface exposed toward the air-side of the photovoltaic module and an inner surface opposite the outer surface and exposed toward the inside of the photovoltaic module.
Subsequently, electrically conducive material layer 220 is prepared, within which connecting circuits 220c is to be obtained. Layer 220 is prepared by placing an aluminium foil of the desired thickness into a plating apparatus from which the air is evacuated by pumps, thereby creating a vacuum. More specifically the pressure within the plating apparatus is caused to drop down to a value of the order of 10-6 torr. One surface of the aluminium foil is then treated with a plasma so as to remove by means of etching the native oxide layer originally present on the aluminium surface. This surface of the aluminium foil or tape undergoing the surface treatment will be in the following referred to as the "first surface". The surface of the aluminium foil or tape opposite the first surface will instead be referred to as the "second surface".
The plasma used for the treatment may comprise a hydrogen plasma or a plasma based on a different gas or gas mixture. The plasma treatment is performed preferably within a vacuum chamber included in the plating apparatus. In this manner, the oxide on the first surface of the aluminium foil is prevented from reforming after being removed by means of the plasma treatment.
After removing the oxide film from the first surface of the aluminium foil treated with a plasma, a protective thin film of a metal is deposited onto the previously treated surface. The metal forming the protective thin film preferably comprises silver or a metal alloy comprising silver.
The deposition of the thin film is preferably performed by means of physical vapour deposition
(PVD).
For instance, the plating apparatus may comprise a crucible into which a predetermined quantity of the metal to be evaporated may be placed in solid form. Thereafter, the crucible within the plating apparatus is heated up, for example, by letting an electrical current flow from one end to the opposite end of the crucible. The metal in the crucible initially melts and subsequently vaporizes or sublimes due to the low pressure within the plating apparatus. While being in vapour phase, the particles of the metal to be deposited move, e.g. upwards, until they impinge onto the aluminium foil surface facing the crucible. This surface, kept at an appropriately low temperature, receives the evaporated material in the form of condensed metal. Preferably, the surface of the aluminium foil facing the crucible is the first surface previously treated with a plasma, i.e. the surface from which the oxide film has been removed.
After hitting the aluminium foil surface closer to the crucible, the vapours of the metal to be deposited condense thereon. The aluminium foil may be kept at an appropriate temperature, for example at room temperature, so that the condensation of the vapours of the metal to be deposited occurs more promptly. The metal to be deposited, initially in the form of a vapour, is thus deposited so as to remain steadily fixed to the aluminium foil, after the metal vapours have impinged onto the foil surface and have condensed. In this manner, a protective film can be formed onto the first surface of the aluminium foil from which the oxide film has been previously removed.
Two or more crucibles may be also used instead of only one, so as to enable simultaneous deposition of more materials and to form a protective film comprising different chemical species, for example a metal alloy, onto the first surface of the aluminium foil. For example, together with or as an alternative to silver, other metals may be deposited such as nickel, tin, cadmium, tantalum, cobalt, or even copper itself, or else an aluminium-copper alloy.
Among the above-mentioned metals, silver is the most advantageous choice due to several reasons. First of all, silver is the metal with the greatest electrical conductivity within the list of metallic materials provided above. Furthermore, silver, being a noble metal, is particularly stable in air, since a surface thereof exposed to the air hardly reacts with gases (oxygen, water vapour, nitrogen, carbon dioxide, etc.) normally present in air at standard pressures. Moreover, silver is compatible with silver-based adhesives (e.g. ECA) typically used in the field.
Thanks to the method herewith proposed, a protective film may thus be deposited comprising more than one metal. Additionally, the present invention enables deposition of multiple protective films comprised of different metals or metal alloys and super imposed to each other. According to the method proposed by the present invention, during the treatment of the first surface of the aluminium foil, the second surface of the foil is preferably free and exposed toward the outside of the foil. In other words, the second surface of the aluminium foil is neither in contact with nor adheres to other surfaces of other elements. Therefore, the aluminium foil is singularly treated during the sequence of plasma etching and deposition of the thin metal film. So, the aluminium foil is neither fixed nor coupled to any substrate or support while the oxide film is removed and a thin metal film is deposited onto the first surface.
This entails considerable advantages. First of all, the aluminium foil does not require any support or substrate during the sequence of plasma etching and deposition onto the first surface. Furthermore, since the second surface is exposed toward the outside, it may also, where required or needed, be treated or modified, for example by means of any one of the well known techniques of surface treatments or by means of a deposition of a metal film, a dielectric film, a polymeric film, etc.
According to an embodiment of the present invention, the sequence of etching and deposition onto the first aluminium surface preferably occurs within a process of the roll-to-roll type.
Instead of a single aluminium foil, an aluminium tape is inserted into the plating apparatus wound so as to form a reel. The aluminium tape has a desired thickness and width. For example, the width of the tape might be of about 1 m.
After evacuating air and establishing a vacuum within the plating apparatus as described above, the aluminium tape unwinds from the starting reel or first reel along a predetermined path within the plating apparatus. At the end of this path, aluminium winds around an arrival reel or second reel, which is, along with the first reel, arranged within the plating apparatus. At a given point of the path between the first and the second reel, the plasma treatment is applied to the first surface of the aluminium tape unwound from the first reel, in order to remove the native oxide film. Subsequently, a thin metal film, preferably silver, is deposited onto the aluminium tape first surface previously treated. Deposition is performed at a point of the aluminium tape unwinding path arranged downstream from the point at which the plasma treatment is applied. The expression "downstream" and "upstream" are understood to be referred to the unwinding direction of the tape, i.e. the direction going from the first to the second reel along the unwinding path of the aluminium tape.
After the protective metal film has been deposited onto the first surface, the aluminium tape is caused to wind in the second reel. After the entire tape is wound in the second reel, the plating apparatus may be opened and the reel may be extracted, together with the aluminium tape whose first surface is covered by the protective metal film.
The process of preparing conductive layer 220, may thus be speeded up and optimized by operating not onto a single aluminium foil but, rather, onto the whole tape, which is to be cut so as to form the foils to be used in the respective photovoltaic modules.
Thanks to the process stage described above, aluminium foils may thus be obtained having a surface protected by a metal film preferably comprising silver. The aluminium foil obtained by the process does not have any insulating oxide layer between the silver surface and the protective metal layer deposited thereon.
Preferably, also in the embodiment described above the second surface of the aluminium tape is free or exposed toward the outside during the sequence of plasma etching and deposition onto the tape first surface. This entails the advantages illustrated above, with reference to the embodiment wherein the first surface of a single aluminium foil is treated with a plasma and subsequently plated.
After the insulating substrate 210 and the electrically conductive material layer 220 have been prepared in the manner described above, the two layers are coupled so as to be firmly fixed to each other. For example, an adhesive or a thermo-adhesive can be used, interposed between the surface of insulating substrate 210 facing the inside of the photovoltaic material and the surface of the conductive material layer 220 facing substrate 210.
Coupling occurs so that the second surface of conductive layer 220 opposite the treated and plated surface faces the inner surface of insulating substrate 210, i.e. the surface of insulating substrate 210 opposite the surface facing the air-side. In this manner the first surface of the aluminium foil, plated with a thin metal film, remains exposed toward the outside.
After coupling, conductive material layer 220 directly adheres to the inner surface of substrate
210. By the expression "first surface directly adhering to the second surface" it is here intended that the first surface is firmly fixed to the second surface, while no further element or layer is present between the first and the second surface except, at most, an adhesive material, a thermo- adhesive material or in general an element enabling adhesion between the first and second surface. Hence, based on the present invention, the conductive material layer 220 is firmly fixed to the inner surface of substrate 210 of the photovoltaic module and, between conductive material layer 220 and the inner surface of substrate 210, no other element or layer is present except, where needed, an adhesive material, a thermo-adhesive material, or the like.
According to an embodiment of the present invention substrate 210 is produced by means of a process of the roll-to-roll type analogous to that described as a possible option for preparing the conductive material layer 220.
Coupling between substrate 210 and conductive material layer 220 is particularly advantageous when performed by means of a roll- to-roll process.
After the two layers or films have been coupled, foils of a desired dimension (for example 1 .7 m x 1 m) may be obtained comprising substrate 210 and conductive layer 220. Immediately after the foils have been so formed, the conductive material layer is usually continuous and does not have any holes, channels or indentations. Thus, immediately after the coupling, electrically conductive layer 220 typically does not yet form a circuit.
The connecting circuit 220c must thus be formed within electrically conductive material layer 220. Connecting circuit 220c may be formed by means of one of the techniques described above, for example optical lithography, mechanical milling, laser ablation, laser contouring, etc. Forming the connecting circuits 220c thus completes the process of producing a back-contact back-sheet according to the present invention.
According to a further embodiment of the present invention, a dielectric material film 240 may be deposited onto the surface of back-contact back-sheet 200 facing the inside of the photovoltaic module after forming the circuit. The dielectric film 240 is thus deposited onto the plated surface of connecting circuit 220c formed in the conductive material layer 220 and onto the surface portion of substrate 210 upon which layer 220 is fixed and left exposed by connecting circuit 220c. The reader is referred to the description provided above with reference to Fig. 3 for the feature and functionality of dielectric film 240 and for some examples of deposition methods. A back-contact back-sheet has thus been formed wherein connecting circuit 220c is formed from aluminium instead of more costly materials such as, for example, copper. The performances of the connecting circuit according to the present invention are guaranteed by a surface treatment comprising a sequence of a plasma etch, followed by the deposition of a thin film of a material not subject to oxidation and resilient such as, for example, silver. Producing the back-contact back-sheet according to the present invention is particularly convenient and cost effective since a roll-to-roll process may be used.
Although the present invention has been described with reference to the embodiments described above, it is clear for the skilled person that several modifications, variations and improvements of the present invention may be made, in view of the teaching described above and within the scope of the attached claims, without departing from the object and the scope of protection of the invention.
Besides that, those aspects which are deemed to be known by experts in the field have not been described in order to not unduly obfuscate the described invention. Consequently, the invention is not limited by the embodiments described above but it is only limited by the scope of protection of the attached claims.

Claims

1. Back-contact back-sheet for a photovoltaic module comprising back-contact solar cells, said back-contact back-sheet comprising:
a substrate (210) having an outer surface exposed toward the air-side of said photovoltaic module and an inner surface opposite said outer surface and facing the inside of said photovoltaic module,
a layer (220) of an electrically conductive material comprising aluminium adapted to be formed as a connecting circuit (220c) to the electrodes of said solar cells and directly adherent to said inner surface of said substrate (210), said layer of conductive material (220) comprising a protective metal film formed on the surface of said layer of conductive material (220) opposite the surface facing said substrate (210).
2. Back-contact back-sheet according to claim 1 , wherein said protective metal film comprises silver or a metal alloy comprising silver.
3. Back-contact back-sheet according to one of claims 1 or 2, wherein said protective metal film has a thickness in the range of 12 nm to 200 nm and, preferably, in the range of 40 nm to 80 nm.
4. Back-contact backs-sheet according to one of claims 1 to 3, wherein said substrate (210) comprises at least one of the following polymers: polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), polyethylene terephthalate (PET), polyethylene naphthalate (PEN).
5. Back-contact back-sheet for solar cells according to one of claims 1 to 4, wherein said substrate (210) comprises a first insulating layer (212) of a first polymeric material having an outer surface exposed toward the air-side of said photovoltaic module and an inner surface opposite said outer surface and a second insulating layer (216) of a second polymeric material coupled to said inner surface of said first insulating layer (212).
6. Back-contact back-sheet according to one of claims 1 to 4, wherein said substrate (210) comprises:
a first insulating layer (212) of a first polymeric material having an outer surface exposed toward the air-side of said photovoltaic module and an inner surface opposite said outer surface, an intermediate layer (214) of a material impermeable to water vapour coupled to said inner surface of said first insulating layer (212), said intermediate layer having a lower surface facing said first insulating layer (212) and an upper surface opposite said lower surface,
a second insulating layer (216) of a second polymeric material coupled to said upper surface of said intermediate layer (214).
7. Back-contact back-sheet according to one of claims 1 to 6 comprising a layer of dielectric material (240) having a lower surface facing said layer of electrically conductive material (220) and said substrate (210) and an upper surface opposite said lower surface, said layer of dielectric material (240) being deposited onto said inner surface of said substrate (210) and said layer of electrically conductive material (220), said layer of dielectric material (240) being adapted to selectively isolate said connecting circuit (220c) formed in said layer of electrically conductive material (220), said layer of dielectric material (240) having a plurality of through-holes (242), each of said through-holes (242) being adapted to leave exposed a predetermined portion of said layer of electrically conductive material (220).
8. Method for producing a back-contact back-sheet for a photovoltaic module comprising back-contact solar cells, said method comprising the following the steps:
making a substrate (210) of the photovoltaic module, said substrate (210) having an outer surface exposed toward the air-side of said photovoltaic module and an inner surface opposite said outer surface and facing the inside of said photovoltaic module; making a layer (220) of an electrically conductive material comprising aluminium adapted to be formed as a connecting circuit (220c) to the electrodes of said solar cells, said step of making said layer of electrically conductive material (220) comprising a plasma etching treatment performed in a vacuum onto a first surface of said layer of electrically conductive material (220), said plasma etching treatment being followed by a deposition of a protective metal film onto said first surface of said layer of electrically conductive material (220), the second surface of said layer of electrically conductive material (220) opposite said first surface being exposed during said step of making said layer of electrically conductive material (220),
coupling said layer of electrically conductive material (220) to said substrate (210), said coupling being performed in such a way that said second surface of said layer of electrically conductive material (220) faces said inner surface of said substrate (210), processing said layer of electrically conductive material (220) so as to form a connecting circuit (220c) to the electrodes of said solar cells, said step of processing said layer of electrically conductive material (220) being performed after said step of coupling said layer of electrically conductive material (220) to said substrate (210).
9. Method according to claim 8, wherein said step of making said substrate (210) is carried out by means of a roll-to-roll process.
10. Method according to one of claims 8 or 9, wherein said step of making said layer of said electrically conductive material (220) is carried out by means of a roll-to-roll process.
1 1. Method according to one of claims 8 to 10, wherein said step of coupling said layer of electrically conductive material (220) to said substrate (210) is carried out by means of a roll-to-roll process.
12. Method according to one of claims 8 to 11 , wherein the plasma used for said plasma etching treatment of said first surface of said layer of electrically conductive material (200) comprise a hydrogen plasma.
13. Method according to one of claims 8 to 12, wherein said film of protective material deposited onto said first surface of said layer of conductive material (220) comprises silver or an alloy comprising silver.
14. Method according to one of claims 8 to 13, wherein said deposition of said protection metal film is carried out in a vacuum by using Physical Vapour Deposition (PVD).
15. Method according to one of claims 8 to 14, wherein said step of processing said layer of conductive material (220) is performed by using at least one of the following techniques: mechanical milling, photolithography, laser ablation, laser contouring.
16. Method according to one of claims 8 to 15 further comprising a step of depositing a layer of dielectric material (240) onto said inner surface of said substrate (210), said step of depositing said layer of dielectric material (240) being performed after said step of coupling said layer of electrically conductive material (220) to said substrate (210).
PCT/IB2013/054434 2012-06-05 2013-05-29 Back-sheet for photovoltaic modules comprising back-contact solar cells WO2013182955A2 (en)

Applications Claiming Priority (6)

Application Number Priority Date Filing Date Title
ITVI2012A000132 2012-06-05
IT000132A ITVI20120132A1 (en) 2012-06-05 2012-06-05 BACKSHEET FOR PHOTOVOLTAIC MODULES INCLUDING CELLS CONTACT REAR
IT000133A ITVI20120133A1 (en) 2012-06-05 2012-06-05 APPLICATION OF THE BACKSHEET ENCAPSTER FOR PHOTOVOLTAIC MODULES USING CELLS CONTACT REAR
ITVI2012A000133 2012-06-05
IT000266A ITVI20120266A1 (en) 2012-10-12 2012-10-12 BACKSHEET FOR PHOTOVOLTAIC MODULES INCLUDING CELLS CONTACT REAR
ITVI2012A000266 2012-10-12

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