WO2019076913A1 - Electro-conductive backsheet for solar cell modules - Google Patents

Electro-conductive backsheet for solar cell modules Download PDF

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Publication number
WO2019076913A1
WO2019076913A1 PCT/EP2018/078273 EP2018078273W WO2019076913A1 WO 2019076913 A1 WO2019076913 A1 WO 2019076913A1 EP 2018078273 W EP2018078273 W EP 2018078273W WO 2019076913 A1 WO2019076913 A1 WO 2019076913A1
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WIPO (PCT)
Prior art keywords
layer
electro
polypropylene
backsheet
conductive
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PCT/EP2018/078273
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French (fr)
Inventor
Franciscus Gerardus Henricus Van Duijnhoven
Robert Janssen
Ian Bennett
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Dsm Ip Assets B.V.
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Publication of WO2019076913A1 publication Critical patent/WO2019076913A1/en

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    • 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
    • H01L31/048—Encapsulation of modules
    • H01L31/0481—Encapsulation of modules characterised by the composition of the encapsulation material
    • 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
    • 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 is directed to an electro-conductive backsheet for back contacted solar cell or photovoltaic modules.
  • the present invention also relates to a process for the manufacturing of the electro-conductive backsheet (ECB) for back contacted solar cell or photovoltaic modules.
  • the present invention further relates to photovoltaic modules comprising the electro-conductive back sheet.
  • Photovoltaic modules are used to generate electrical energy from sunlight. Photovoltaic modules are an important source of renewable energy. They include solar cells that release electrons when exposed to sunlight. The solar cells, which are usually semiconductor materials that may be fragile, are typically
  • the encapsulated solar cells are generally further protected by glass or by another outer layer that is resistant to weathering, abrasion, or other physical insults.
  • a photovoltaic module has a front surface protection sheet disposed on the side on which sunlight is incident, to protect the surface.
  • the module also has a solar cell rear surface protection sheet (backsheet) disposed on the opposite side to protect power generation cells.
  • backsheet solar cell rear surface protection sheet
  • Such sheets are required to provide weatherability, water resistance, heat resistance, moisture proof properties, gas barrier properties and to minimize deterioration in the long-term performance of solar cell modules.
  • laminates of different polymers layers have been used. Examples of such polymer layers include polypropylene, polyvinyl chloride, polyesters, fluororesins, and acrylic resins.
  • the backsheet may comprise a metal layer to provide an electro- conductive backsheet.
  • Electro-conductive backsheets are well known in the art. Usually electro-conductive backsheets comprise a laminate of a metal foil such as copper and/ or aluminium, an adhesive layer and at least a polymer layer.
  • the metal foil may comprise a pattern. The pattern can be obtained through known patterning
  • the polymer layer may be a multilayer to provide a multilayer backsheet.
  • Electro-conductive backsheets are disclosed in the prior art such as for example in EP2720519.
  • the patent application describes an ECB comprising a metal layer such as copper, an adhesive layer such as an epoxy resin and a polymer layer such as PET.
  • a disadvantage is that both the hydrolytic and UV resistance of PET is rather poor, limiting the durability and hence service life of PV modules.
  • the backsheet may be partly damaged by the patterning process leading to detonation of the electrical insulation properties of the backsheet polymeric layer facing the cells.
  • Another disadvantage is that after patterning there are areas where no metal is present but either the adhesive layer or the polymer layer facing the cells is present. In those areas, the adhesive layer or polymeric layer is in direct contact with a back-encapsulant which is traditionally EVA. The adhesion between the back- encapsulant and polymeric layer often limited and may lead to delamination during lifetime of the module.
  • PV modules often comprise a polymer layer facing the cells which melts during lamination at 130 - 140 °C. Melting of this layer is intended to obtain/increase adhesion with the back- encapsulant.
  • the patterned metal layer will start drifting during lamination. Drifting of the metal pattern results in electrical short circuits and imperfections in contacting the cells to the patterned metal layer.
  • the polymer layer facing the cells is meant that the polymer layer is situated at the cell side in a PV module.
  • figure 1 the order of the layers in the conductive backsheet according to the present invention is shown.
  • an electro- conductive backsheet comprising: (1 ) a metal layer (2) an adhesive layer and (3) a backsheet whereby the backsheet comprises a polypropylene layer, with a melting point of at least 140 °C whereby the polypropylene layer connects to the metal layer via the adhesive layer.
  • the electro-conductive back-sheet provides improved adhesive properties between the metal layer and polypropylene layer and between the back-encapsulant and the adhesive layer especially in case of a patterned metal layer.
  • the electro-electro-conductive back-sheet has improved processing stability during lamination and improved insulating properties. Hence breakthrough/shot circuits are prevented and the efficiency and lifetime of the resulting back contacted PV module is improved resulting in higher energy yields during service life.
  • the polypropylene layer which connects to the metal layer via the adhesive layer comprises a polypropylene with a melting point above 140°C.
  • Examples of polypropylenes with a melting point of at least 140 °C are polypropylene
  • Block copolymers of polypropylene have co-monomer units arranged in blocks (in a regular pattern) and contain for example between 5 wt% to 15 wt% ethylene.
  • Random copolymers of polypropylene have co- monomer units arranged in irregular or random patterns along the polypropylene chain. They are usually incorporated anywhere and contain between 1 wt% to 7wt % ethylene. A person skilled in the art will be able to select a polypropylene homopolymer or a polypropylene copolymer with an ethylene content such that the melting point will be above 140 °C.
  • the polypropylene layer comprises polypropylene impact copolymers.
  • Polypropylene impact copolymers are thermoplastic resins produced through the polymerization of propylene and ethylene by using Ziegler Natta catalysts. Their synthesis consists of a heterophasic amorphous structure inside a semi- crystalline PP homopolymer matrix.
  • the impact polypropylene copolymers may be produced in a two-reactor system to yield a blend of polypropylene
  • the layer of polypropylene which connects to the metal layer via the adhesive layer, has a melting point above 140 degrees °C to prevent melting even during lamination of the back contacted PV module.
  • the polypropylene layer preferably has a thickness of maximum 75 microns. More preferably it has a thickness of maximum 40 microns, even more preferably it has a thickness of maximum 30 microns.
  • the polypropylene layer may further comprise additives such as fillers, stabilizers, colorants and/or pigments.
  • additives such as UV stabilizers, UV absorbers, anti-oxidants, thermal stabilizers and/or hydrolysis stabilizers.
  • the polypropylene layer and optionally other polymeric layer(s) may comprise from 0.05-10 wt.%, additives more preferably from 1 - 5 wt.% additives based on the total weight of the polymer.
  • the polypropylene layer comprises a white inorganic filler such as Ti02, ZnO or ZnS to increase reflection and improve UV resistance.
  • Black pigments such as carbon black or Fe2C>3 can also be added for increased UV resistance and increased thermal conductivity. It will moreover lower the temperature of the photovoltaic module and hence increase the efficiency of the module (0.5% per 1 °C).
  • the metal layer for example comprises a metal with a high electrical conductivity such as molybdenum, aluminum, copper, silver, gold, copper coated aluminum or an alloy of copper and aluminum.
  • the metal layer may have a thickness ranging from 20 ⁇ to 100 ⁇ . Preferably the thickness ranges from 50 ⁇ to 90 ⁇ . More preferably the thickness ranges from 60 ⁇ to 80 ⁇ .
  • the metal layer preferably comprises copper, aluminum or copper coated aluminum. More preferably the metal layer is copper or copper coated aluminum.
  • the thickness of the copper layer more preferably ranges from 150 nm to 50 ⁇ , more preferably the thickness of the copper layer ranges from 10 ⁇ to 45 ⁇ .
  • the thickness of the copper layer depends on the way the copper is coated to the aluminum.
  • the copper may be applied to the aluminum by physical vapor deposition (PVD) or by cold spraying (Cs). Preferably the copper is applied via cold spray.
  • the metal layer preferably comprises a pattern.
  • Such pattern can be obtained through known patterning technologies. Examples of known patterning technologies given as an indication without being limiting are mechanical milling, chemical etching, laser ablation and die cutting. Laser ablation, milling or die cutting are preferably used. More preferably die cutting is used.
  • the adhesive layer may comprise an epoxy polymer, an acrylate containing polymer or a polyurethane adhesive.
  • a polyurethane adhesive layer Preferably it comprises a polyurethane adhesive layer.
  • the polyurethane is cured from a waterborne, solvent- based or solvent less polyester resin with an isocyanate crosslinking agent.
  • solvent based polyester resins Most preferred are solvent based polyester resins. Examples of these kind of polyurethanes are available under the tradename Adcote® from Dow Chemicals or NeoRez® from DSM.
  • the backsheet as mentioned herein means a backsheet that is composed of at least a layer of polypropylene with a melting point of at least 140 °C that connects to the metal layer via the adhesive layer.
  • the backsheet is called a monolayer backsheet.
  • the backsheet is called a multilayer backsheet.
  • the electro- conductive backsheet preferably comprises at least 2 and up to 8 polymeric layers.
  • the backsheet preferably comprises 2-5 polymeric layers. These layers are preferably co- extruded to yield defect free multilayer structures.
  • the polymeric layer(s) as mentioned herein comprise thermoplastic or thermosetting polymers.
  • a thermoplastic polymer is a polymer that becomes pliable or moldable above a specific temperature and solidifies upon cooling.
  • Thermoplastics differ from thermosetting polymers in that thermosetting polymers form irreversible chemical bonds during a curing process. Thermosets do not melt but decompose and do not reform upon cooling. Thermoplastic polymers are preferred.
  • the backsheet may optionally comprise other polymeric layers beside the polypropylene layer that connects to the metal layer (1 ) via the adhesive layer (2) such as a tie layer (4) and/or a core layer (5) and/or a rear layer (6).
  • the order of these layers or their position in the electro-conductive backsheet is shown in figure 2.
  • These polymeric layers may further comprise inorganic fillers or additives as referred to in above mentioned paragraph.
  • the tie layer for example comprises a maleic anhydride grafted polyolefine such as maleic anhydride grafted polyethylene or maleic
  • the adhesive layer comprises a maleic anhydride grafted polyolefine such as a maleic anhydride grafted polyethylene or a maleic anhydrate grafted polypropylene.
  • the adhesive layer may also comprise polyethylene (PE), preferably "polar" polyethylene, i.e. ethylene copolymerized with polar co monomers chosen from vinyl acetate, acrylic and methacrylic ester (methylacrylate, ethylacrylate, butylacrylate, ethylhexylacrylate.
  • PE polyethylene
  • polar polyethylene i.e. ethylene copolymerized with polar co monomers chosen from vinyl acetate, acrylic and methacrylic ester (methylacrylate, ethylacrylate, butylacrylate, ethylhexylacrylate.
  • This adhesive layer may connect the polypropylene layer which connects the metal layer (1 ) via the adhesive layer (2) with the structured layer or the weather resistant layer.
  • the core layer may comprise a thermoplastic polyolefin (TPO).
  • TPO may comprise a flexible polypropylene (FPP) mechanical or reactor blends of PP resins (homo or copolymer) with EPR rubber (ethylene propylene rubber), like Hifax CA 10 A, Hifax CA 12, Hifax CA7441A, supplied by LyondellBasell, a mechanical FPP blends of PP resins with elastomer PP resins (for example supplied by Dow under the trade name Versify 2300.01 or 2400.01 ), a mechanical FPP blends of PP (preferably random copolymer of propylene (RCP) with ethylene and possibly other olefins) with LLDPE (linear low densitiy polyethylene) or VLDPE (very low density polyethylene) plastomers (like Exact 0201 or Exact 8201 supplied by Dex Plastomers),
  • FPP flexible polypropylene
  • EPR rubber ethylene propylene rubber
  • polymerization FPP blends of PP blocks with PE blocks olefin block copolymers (OBC) resins of crystalline PE parts (blocks) and amorphous copolymerized PE parts (blocks) providing softness and high DSC melting temperature (plus or minus 120 °C), like INFUSETM olefin block copolymers supplied by Dow, polyethylene plastomers (VLDPE) or LLDPE, i.e. PE obtained by copolymerization of ethylene with short-chain alpha- olefins (for example, 1 -butene, 1 -hexene and 1 -octene), with a density of less than 925 kg/m 3 (ISO 1 183).
  • OBC olefin block copolymers
  • VLDPE polyethylene plastomers
  • LLDPE LLDPE
  • the core layer may comprise an UV stabilized polyethylene terephthalate (PET), PET, polybutylene terephthalate PBT or PBT with an impact-modifying component, such as EP rubber, EPM rubber, EPDM rubber or SEBS.
  • PET polyethylene terephthalate
  • PBT polybutylene terephthalate
  • EP rubber such as EP rubber, EPM rubber, EPDM rubber or SEBS.
  • the core layer may comprise polypropylene homopolymers and/or polypropylene copolymers.
  • the copolymers are further divided into block copolymers and random copolymers.
  • Block copolymers of polypropylene have co-monomer units arranged in blocks (in a regular pattern) and contain for example between 5 wt% to 15 wt% ethylene.
  • Random copolymers of polypropylene have co-monomer units arranged in irregular or random patterns along the
  • polypropylene chain are usually incorporated anywhere and contain between 1 wt% to 7wt % ethylene.
  • the polypropylene layer comprises polypropylene impact copolymers.
  • the impact polypropylene copolymers may be produced in a two-reactor system to yield a blend of polypropylene homopolymer with an ethylene-propylene rubber (EPR).
  • EPR ethylene-propylene rubber
  • the rear layer of the backsheet may comprise a polyamide or fluoro- resin such as PTFE or PVDF.
  • a polyamide layer can act as an oxygen barrier layer.
  • the polyamide is preferably selected from the group consisting of polyamide 6;
  • polyamide 6,6 polyamide 4,6; polyamide 6,10; polyamide 6,12; polyamide 6,14;
  • polyamide 10 polyamide 9,12; polyamide 9,13; polyamide 9,14; polyamide 9,15;
  • polyamide 6,16 polyamide 10,10; polyamide 10,12; polyamide 10,13; polyamide 10,14; polyamide 12,10; polyamide 12,12; polyamide 12,13 or polyamide 12,14; polyamide 4,10, polyamide 5,10 or polyamide 6,10 or any mixtures thereof.
  • polyamides are selected with limited moisture uptake such as polyamide 1 1 or 12. Low moisture uptake guarantees good electrical insulation also in high humidity environments.
  • the PA layer can be rubber modified to further reduce the moisture uptake and increase the flexibility.
  • the rear layer may also comprise an UV stabilized polyethylene
  • PET polybutylene terephthalate
  • PBT polybutylene terephthalate
  • PBT polybutylene terephthalate
  • an impact-modifying component such as EP rubber, EPM rubber, EPDM rubber or SEBS.
  • the core and the rear layer typically comprise different polymeric materials.
  • the core layer may comprise PET and the rear layer may comprise UV stabilized PET.
  • the core layer may comprise polypropylene homopolymer and/or polypropylene copolymer and the rear layer may comprise a polyamide.
  • the core layer may comprise a polypropylene homopolymer and/or polypropylene copolymer and the rear layer may comprise a rubber modified PBT.
  • the thickness of the electro-electro-conductive back-sheet is preferably from 0.15 to 1 mm, more preferably from 0.15 to 0.8 mm, even more preferably from 0.15 to 0.75 mm.
  • the present invention also relates to a process for the manufacturing of the electroconductive backsheet comprising the following steps;
  • the laminating step a) is performed using a process well known to those skilled in the art by coating either or both the metal layer and polypropylene layer with the adhesive layer, drying of the adhesive layer and subsequent joining of the coated metal layer and/or polypropylene layer in for instance a roll-to-roll process. Curing of the adhesive layer is preferable performed at a temperature below 80 °C to avoid curling of the electro-conductive backsheet structure due to the differences in thermal expansion coefficient between the metal layer and the polypropylene layer.
  • the process for the manufacturing of an electro-conductive backsheet comprises the following steps:
  • the adhesive layer is laminated and/or extruded with a polypropylene layer and optionally other polymeric layers to provide a polypropylene and optionally other polymeric layers connected to an adhesive layer
  • the present invention also relates to a photovoltaic module comprising the electro-conductive backsheet according to the present invention.
  • a photovoltaic module (abbreviated PV module) comprises at least the following layers in order of position from the front sun-facing side to the back non-sun-facing side: a transparent pane (representing the front sheet), a front encapsulant layer, a solar cell layer, a back encapsulant layer, and the electro-conductive back sheet comprising a metal layer (1 ), an adhesive layer (2) to connect the metal layer to the backsheet wherein the backsheet comprises a polypropylene layer (3) with a melting point of at least 140 °C which connects to the metal layer via the adhesive layer.
  • the backsheet may optionally comprise further polymeric layers such as a tie layer (4), a core layer (5) and/or a rear layer (6). A further description of these optional polymeric layers is given in the paragraphs above.
  • the front sheet is typically a glass plate or a rigid polymer sheet.
  • the front and back encapsulant used in solar cell modules are designed to encapsulate and protect the fragile solar cells.
  • the "front side” corresponds to a side of the photovoltaic cell irradiated with light, i.e. the light-receiving side, whereas the term “backside” corresponds to the reverse side of the light- receiving side of the photovoltaic cells.
  • Suitable encapsulants typically possess a combination of
  • ethylene/vinyl acetate copolymers are the most widely used encapsulants.
  • Suitable encapsulants include polyolefins, such as
  • Suitable ethylene copolymers include those comprising copolymerized units of ethylene and carboxylic acids such as (meth)acrylic acids (including esters thereof (i.e., acrylates) and salts thereof (i.e., ionomers)), and combinations of two or more thereof.
  • this may be a low density polyethylene, linear low density polyethylene, very low density polyethylene, ultra-low density polyethylene, medium density polyethylene, high density polyethylene, metallocene catalyzed polyethylene, and combinations of two or more of these polyethylenes.
  • the present invention further relates to a process for preparing such solar cell module, which process comprises (a) providing an assembly comprising all the polymer layers recited above and (b) laminating the assembly to form the solar cell module.
  • the laminating step of the process may be conducted by subjecting the assembly to heat and optionally vacuum or pressure.
  • Figure 1 shows the order of the metal layer (1 ), the adhesive layer (2) and the backsheet comprising the polypropylene layer (3).
  • Figure 2 shows the order of the layers in a backsheet comprising the metal layer (1 ), the adhesive layer (2) and the backsheet comprising the polypropylene layer (3) and optionally more polymeric layers such as a tie layer (4), a core layer (5) and/or a rear layer(6).
  • Figure 3 shows the average pattern displacement which is measured by determining the difference in distances of the trenches of the pattern before and after lamination.
  • Back-sheet 1 having a thickness of 306 micron comprising of a layer facing the cells of PET from Mitsubishi Hostphan RN50-250 with a thickness of 250 microns and laminated with Adcote having a thickness of 6 microns to stabilized-PET from Mitsubishi Hostphan RN50-250 with a thickness of 250 microns and laminated with Adcote having a thickness of 6 microns to stabilized-PET from Mitsubishi Hostphan RN50-250 with a thickness of 250 microns and laminated with Adcote having a thickness of 6 microns to stabilized-PET from
  • Back-sheet 2 B10 from DSM Sunshine having a thickness of 360 microns being a coextruded multilayer film comprising a layer facing the cells which layer comprises a LLDPE/EVA blend with a melting point of 105 °C.
  • the other layers are based on a PP core layer and PA12 being the rear layer.
  • Back-sheet 3 (BS3) B10NE from DSM Sunshine having a thickness of 360 microns being a coextruded multilayer film comprising a layer facing the cells which layer comprising of polypropylene with a melting point of at least 140 °C and additives.
  • the other layers are based on PP core layer and PA12 being the rear layer.
  • Metal layer (trial 1 )
  • Electro-conductive back-sheets (ECB) (triaH ):
  • Electro-conductive back-sheets ECB1/ECB2 and ECB3 were produced by lamination. Lamination was performed using a process well known to those skilled in the art by coating both the metal layer and the cell facing layer of the back-sheet with the Adcote adhesive layer, drying of the adhesive layer for 15 minutes at room temperature and subsequent joining of the coated metal layer and/or cell-facing layer of the back-sheet polypropylene layer in for instance a roll-to-roll process. Curing of the adhesive layer was performed at room temperature for 3 weeks:
  • ECB1 Cu-Adcote-BS1 ; ECB2: Cu-Adcote-BS2; ECB3: Cu-Adcote-BS3 Patterning of the electro-conductive back-sheets was performed by mechanical milling.
  • 2x2 mini PV-modules were produced from the ECB1/ECB2/ECB3 through heat- lamination under vacuum at 150 °C for 15 minutes using EVA as encapsulant.
  • Damp-heat ageing the elongation-at-break of the different electro-conductive back- sheets was determined before and after exposure for 1000 hrs to damp-heat (85 °C, 85% RH). The elongation-at-break was determined according to ISO 527.
  • Table A shows that the hydrolytic resistance of ECBs based on PP and PA12 is improved compared to PET-based ECBs
  • Table B shows that the UV resistance of ECBs based on PP and PA12 is improved compared to PET-based ECBs.
  • Peel strength peel strength between back-encapsulant and a mechanically milled area of the electro-conductive back-sheet was determined according to DIN EN 28510-1 .
  • Breakdown voltage is determined: in the middle of a 15x15cm sample an area of 10x10cm was mechanically milled. The breakdown voltage of the milled area was determined according to IEC 60243-1 and compared with the breakdown voltage of the initial back-sheet upon which the ECB was based. Comparison Breakdown voltage difference BS and milled area ECB [kV]
  • Table C shows that there is no drop in breakdown voltage after milling for the electro- conductive back-sheet having the polypropylene layer facing the cells especially compared to the other ECBs. Also, the initial breakdown voltage is higher allowing higher system voltages.
  • PE layer comprises 70 parts of LLDPE, 20 parts of EVA, 10 parts of Ti02 and 0.5 parts of an ultraviolet stabilizer, blended. Thickness of the PE layer is 50 ⁇ PP layer comprises 90 parts of copolymerized PP, 10 parts of Ti02 and 0.3 parts of Irganox 1010, blended. Thickness of the PP layer is 250 ⁇
  • PA12 layer comprises 100 parts of PA12, 0.5 parts of Tinuvin770, 0.3 parts of Irganox B225 and 10 parts of Ti02, blended. Thickness of the PA12 layer is 25 ⁇
  • Tie-layer between the PP and PE, between the PP and PA12 consists of 100 parts of maleic anhydride grafted polypropylene, thickness of the tie layer is 25 ⁇
  • a backsheet is prepared by a multilayer co-extrusion process whereby the pellets of the respective layers are added to a twin-screw extruder, melt-extruded at high temperature, and pushed through adapters through and a die, cooled by a cooling roller and shaped to manufacture the multi-layer back sheet. Subsequently the metal layer is coated using a K Control Benchtop Laboratory Laminator with 25-micron Adcote A3305 polyester adhesive in combination with CR857 isocyanate co-reactant (both available from Dow) resulting in an Adcote-coated metal substrate. The coating is left to dry for 15 minutes.
  • Backsheets prepared by co-extrusion and a reference PVF- PET-PVF Krzz Akasol® PTL 3 HR 1000V backsheet are corona treated on the side to be adhered to the Adcote-coated metal substrate using a Tigres 600W corona treater.
  • the corona treated backsheets are joined with Adcote-side of the Adcote- coated metal substrate using a Rycobel Benchtop Laboratory Laminator.
  • the Adcotejs allowed to cure in an oven at 60 °C for 2 days.
  • the electro-conductive backsheets are patterned using an Eurotron IBP machine such that the final patterned electro-conductive backsheet is obtained. Compositions of the different layers in the electro-conductive backsheets are given in tables 3-5.
  • 2x2 back-contacted mini PV-modules were made using an Eurotron Euromini laboratory tool through laminating for 15 minutes at 145°C a stack of Scheuten: Super white 4 mm thick glass, a STR Photocap 15580P/UF (not punched) front encapsulant, JA Solar back-contact cells, a STR Photocap 15580P/UF (punched) back-encapsulant with EMS DB1588-4 conductive adhesive in the punched holes for connecting the cells to the metal substrate of the electro-conductive backsheet.
  • the cells and conductive adhesive are omitted. Measurements (Trial 2)
  • Peel strength is measured at 90 °C angle using a Zwick Z050 tensile tester at 100 mm/min between the metal layer and the backsheet, and between the backsheet and the EVA back-encapsulant.
  • the average pattern displacement is measured by determining the difference in distances of the trenches of the pattern before and after lamination as indicated in Figure 2.
  • FF [%] is a parameter which, in conjunction with Voc and Isc, determines the maximum power from a solar cell.
  • the FF is defined as the ratio of the maximum power from the solar cell to the_product of Voc and Isc.
  • the change in power and FF are determined as an average of 4 2x2 mini-modules of the same build.
  • the peel strength between the metal substrate and the backsheet, and between the backsheet and the encapsulant is given.
  • the peel strength between the backsheet and the EVA represents the peel strength in the trenches of the pattern where the metal and (part of the) adhesive are removed by milling.
  • PVD physical vapor
  • Table 1 shows the improved adhesive properties between the metal layer and PP-layer compared to using a backsheet based on PE with a melting point well below 140 °C. Also, the adhesive properties between the back-encapsulant and the backsheet are improved compared to the adhesive properties using a backsheet with a PE layer having a melting point of well below 140 °.
  • Table 2 shows that the processing stability during lamination improves when replacing PE with a melting point of well below 140 °C by a PP-layer having a melting point of well above 140 °C.
  • Table 3 shows that the improved adhesive properties between the metal layer and polypropylene layer, and between the back-encapsulant and the adhesive layer in combination with the improved processing stability during lamination lead to improved insulating properties and hence improved electrical performance of the back-contacted PV-modules. This results in higher output of the modules over time and hence increased lifetime. It also shown that the electrical performance and hence lifetime is also better compared to a backsheet that is not based on PP.

Abstract

The present invention relates to an electro-conductive backsheet comprising: (1) a metal layer (2) an adhesive layer, (3) a backsheet whereby the backsheet comprises a polypropylene layer, with a melting point of at least 140 ºC and additives, and the polypropylene layer connects to the metal layer via the adhesive layer. The polypropylene layer is preferably isotactic polypropylene and has a thickness equal or below 75 microns. The additives are chosen from the group consisting of fillers, stabilizers, colorants and/or pigments. The metal layer is chosen from a copper layer, aluminium layer or a copper-coated aluminum layer. Preferably the metal layer is patterned. The adhesive layer is selected from the group consisting of an epoxy polymer, an acrylate containing polymer or a polyurethane adhesive. Preferably the polyurethane is cured from a waterborne, solvent-based or solvent less polyester resin with an isocyanate crosslinking agent. The present invention further relates to a photovoltaic module comprising the electro-conductive backsheet. The invention also relates to a process for the manufacturing of an electro-conductive backsheet wherein (a) the metal layer and the adhesive layer are laminated into a metal-adhesive laminate and (b) a polypropylene layer and optionally other polymeric layers are provided (c) co-extrusion of the metal-adhesive laminate with the polypropylene layer and the optional other polymeric layers (d). Optional subsequent patterning of the metal layer.

Description

ELECTRO-CONDUCTIVE BACKSHEET FOR SOLAR CELL MODULES
The present invention is directed to an electro-conductive backsheet for back contacted solar cell or photovoltaic modules. The present invention also relates to a process for the manufacturing of the electro-conductive backsheet (ECB) for back contacted solar cell or photovoltaic modules. The present invention further relates to photovoltaic modules comprising the electro-conductive back sheet.
Photovoltaic modules are used to generate electrical energy from sunlight. Photovoltaic modules are an important source of renewable energy. They include solar cells that release electrons when exposed to sunlight. The solar cells, which are usually semiconductor materials that may be fragile, are typically
encapsulated in polymeric materials that protect them from physical shocks and scratches. The encapsulated solar cells are generally further protected by glass or by another outer layer that is resistant to weathering, abrasion, or other physical insults.
A photovoltaic module has a front surface protection sheet disposed on the side on which sunlight is incident, to protect the surface. The module also has a solar cell rear surface protection sheet (backsheet) disposed on the opposite side to protect power generation cells. Such sheets are required to provide weatherability, water resistance, heat resistance, moisture proof properties, gas barrier properties and to minimize deterioration in the long-term performance of solar cell modules. To obtain backsheets with the above properties laminates of different polymers layers have been used. Examples of such polymer layers include polypropylene, polyvinyl chloride, polyesters, fluororesins, and acrylic resins.
The backsheet may comprise a metal layer to provide an electro- conductive backsheet. Electro-conductive backsheets are well known in the art. Usually electro-conductive backsheets comprise a laminate of a metal foil such as copper and/ or aluminium, an adhesive layer and at least a polymer layer. The metal foil may comprise a pattern. The pattern can be obtained through known patterning
technologies such as mechanical milling, chemical etching, laser ablation or die cutting. The polymer layer may be a multilayer to provide a multilayer backsheet.
Electro-conductive backsheets (ECB) are disclosed in the prior art such as for example in EP2720519. The patent application describes an ECB comprising a metal layer such as copper, an adhesive layer such as an epoxy resin and a polymer layer such as PET. A disadvantage is that both the hydrolytic and UV resistance of PET is rather poor, limiting the durability and hence service life of PV modules.
Another disadvantage is that when the metal layer is patterned, the backsheet may be partly damaged by the patterning process leading to detonation of the electrical insulation properties of the backsheet polymeric layer facing the cells.
Another disadvantage is that after patterning there are areas where no metal is present but either the adhesive layer or the polymer layer facing the cells is present. In those areas, the adhesive layer or polymeric layer is in direct contact with a back-encapsulant which is traditionally EVA. The adhesion between the back- encapsulant and polymeric layer often limited and may lead to delamination during lifetime of the module.
Still another disadvantage is that standard PV modules often comprise a polymer layer facing the cells which melts during lamination at 130 - 140 °C. Melting of this layer is intended to obtain/increase adhesion with the back- encapsulant. When an ECB is based on such back-sheets, the patterned metal layer will start drifting during lamination. Drifting of the metal pattern results in electrical short circuits and imperfections in contacting the cells to the patterned metal layer.
Delamination will limit the service life of the photovoltaic module due to for instance increased water and air/oxygen ingress at the laminated interface and is therefore undesirable.
There is thus a need to develop an ECB with improved durability and service life when used in photovoltaic modules.
It is a further object of the present invention to provide an ECB with improved adhesion between the back-encapsulant in patterned areas and the adhesive layer facing the cells. There is also a need to improve the adhesion between adhesive layer and polymer layer facing the cells and/or the adhesion between polymer layer facing the cells and other polymer layers in the backsheet. With the polymer layer facing the cells is meant that the polymer layer is situated at the cell side in a PV module. In figure 1 , the order of the layers in the conductive backsheet according to the present invention is shown.
The object of the present invention is achieved in that an electro- conductive backsheet is provided comprising: (1 ) a metal layer (2) an adhesive layer and (3) a backsheet whereby the backsheet comprises a polypropylene layer, with a melting point of at least 140 °C whereby the polypropylene layer connects to the metal layer via the adhesive layer.
It has surprisingly been found that the electro-conductive back-sheet provides improved adhesive properties between the metal layer and polypropylene layer and between the back-encapsulant and the adhesive layer especially in case of a patterned metal layer. Moreover, the electro-electro-conductive back-sheet has improved processing stability during lamination and improved insulating properties. Hence breakthrough/shot circuits are prevented and the efficiency and lifetime of the resulting back contacted PV module is improved resulting in higher energy yields during service life.
The polypropylene layer which connects to the metal layer via the adhesive layer comprises a polypropylene with a melting point above 140°C. Examples of polypropylenes with a melting point of at least 140 °C are polypropylene
homopolymers or polypropylene copolymers. The copolymers are further divided into block copolymers and random copolymers. Block copolymers of polypropylene have co-monomer units arranged in blocks (in a regular pattern) and contain for example between 5 wt% to 15 wt% ethylene. Random copolymers of polypropylene have co- monomer units arranged in irregular or random patterns along the polypropylene chain. They are usually incorporated anywhere and contain between 1 wt% to 7wt % ethylene. A person skilled in the art will be able to select a polypropylene homopolymer or a polypropylene copolymer with an ethylene content such that the melting point will be above 140 °C. Preferably the polypropylene layer comprises polypropylene impact copolymers. Polypropylene impact copolymers are thermoplastic resins produced through the polymerization of propylene and ethylene by using Ziegler Natta catalysts. Their synthesis consists of a heterophasic amorphous structure inside a semi- crystalline PP homopolymer matrix. Typically, the impact polypropylene copolymers may be produced in a two-reactor system to yield a blend of polypropylene
homopolymer with an ethylene-propylene rubber (EPR). These impact polypropylene copolymers are commercially available from for example Lyondellbasell.
The layer of polypropylene, which connects to the metal layer via the adhesive layer, has a melting point above 140 degrees °C to prevent melting even during lamination of the back contacted PV module. The polypropylene layer preferably has a thickness of maximum 75 microns. More preferably it has a thickness of maximum 40 microns, even more preferably it has a thickness of maximum 30 microns.
The polypropylene layer may further comprise additives such as fillers, stabilizers, colorants and/or pigments. Examples of the additives are UV stabilizers, UV absorbers, anti-oxidants, thermal stabilizers and/or hydrolysis stabilizers. When such additives stabilizers are used, the polypropylene layer and optionally other polymeric layer(s) may comprise from 0.05-10 wt.%, additives more preferably from 1 - 5 wt.% additives based on the total weight of the polymer.
Preferably the polypropylene layer comprises a white inorganic filler such as Ti02, ZnO or ZnS to increase reflection and improve UV resistance. Black pigments such as carbon black or Fe2C>3 can also be added for increased UV resistance and increased thermal conductivity. It will moreover lower the temperature of the photovoltaic module and hence increase the efficiency of the module (0.5% per 1 °C).
The metal layer for example comprises a metal with a high electrical conductivity such as molybdenum, aluminum, copper, silver, gold, copper coated aluminum or an alloy of copper and aluminum. The metal layer may have a thickness ranging from 20 μηι to 100 μηι. Preferably the thickness ranges from 50 μηι to 90 μηι. More preferably the thickness ranges from 60 μηι to 80 μηι.
The metal layer preferably comprises copper, aluminum or copper coated aluminum. More preferably the metal layer is copper or copper coated aluminum. The thickness of the copper layer more preferably ranges from 150 nm to 50 μηι, more preferably the thickness of the copper layer ranges from 10 μηι to 45 μηι. The thickness of the copper layer depends on the way the copper is coated to the aluminum. The copper may be applied to the aluminum by physical vapor deposition (PVD) or by cold spraying (Cs). Preferably the copper is applied via cold spray.
The metal layer preferably comprises a pattern. Such pattern can be obtained through known patterning technologies. Examples of known patterning technologies given as an indication without being limiting are mechanical milling, chemical etching, laser ablation and die cutting. Laser ablation, milling or die cutting are preferably used. More preferably die cutting is used.
The adhesive layer may comprise an epoxy polymer, an acrylate containing polymer or a polyurethane adhesive. Preferably it comprises a polyurethane adhesive layer. More preferably the polyurethane is cured from a waterborne, solvent- based or solvent less polyester resin with an isocyanate crosslinking agent. Most preferred are solvent based polyester resins. Examples of these kind of polyurethanes are available under the tradename Adcote® from Dow Chemicals or NeoRez® from DSM.
The backsheet as mentioned herein means a backsheet that is composed of at least a layer of polypropylene with a melting point of at least 140 °C that connects to the metal layer via the adhesive layer. In case that one layer is present, the backsheet is called a monolayer backsheet. In case that more polymeric layers are present, the backsheet is called a multilayer backsheet. The electro- conductive backsheet preferably comprises at least 2 and up to 8 polymeric layers. The backsheet preferably comprises 2-5 polymeric layers. These layers are preferably co- extruded to yield defect free multilayer structures.
The polymeric layer(s) as mentioned herein comprise thermoplastic or thermosetting polymers. A thermoplastic polymer is a polymer that becomes pliable or moldable above a specific temperature and solidifies upon cooling. Thermoplastics differ from thermosetting polymers in that thermosetting polymers form irreversible chemical bonds during a curing process. Thermosets do not melt but decompose and do not reform upon cooling. Thermoplastic polymers are preferred.
The backsheet may optionally comprise other polymeric layers beside the polypropylene layer that connects to the metal layer (1 ) via the adhesive layer (2) such as a tie layer (4) and/or a core layer (5) and/or a rear layer (6). The order of these layers or their position in the electro-conductive backsheet is shown in figure 2. These polymeric layers may further comprise inorganic fillers or additives as referred to in above mentioned paragraph.
The tie layer for example comprises a maleic anhydride grafted polyolefine such as maleic anhydride grafted polyethylene or maleic
anhydride grafted polypropylene, an ethylene-acrylic acid copolymer or an
ethyleneacrylic ester-maleic anhydride terpolymer. Preferably, the adhesive layer comprises a maleic anhydride grafted polyolefine such as a maleic anhydride grafted polyethylene or a maleic anhydrate grafted polypropylene. The adhesive layer may also comprise polyethylene (PE), preferably "polar" polyethylene, i.e. ethylene copolymerized with polar co monomers chosen from vinyl acetate, acrylic and methacrylic ester (methylacrylate, ethylacrylate, butylacrylate, ethylhexylacrylate. This adhesive layer may connect the polypropylene layer which connects the metal layer (1 ) via the adhesive layer (2) with the structured layer or the weather resistant layer.
The core layer may comprise a thermoplastic polyolefin (TPO). The TPO may comprise a flexible polypropylene (FPP) mechanical or reactor blends of PP resins (homo or copolymer) with EPR rubber (ethylene propylene rubber), like Hifax CA 10 A, Hifax CA 12, Hifax CA7441A, supplied by LyondellBasell, a mechanical FPP blends of PP resins with elastomer PP resins (for example supplied by Dow under the trade name Versify 2300.01 or 2400.01 ), a mechanical FPP blends of PP (preferably random copolymer of propylene (RCP) with ethylene and possibly other olefins) with LLDPE (linear low densitiy polyethylene) or VLDPE (very low density polyethylene) plastomers (like Exact 0201 or Exact 8201 supplied by Dex Plastomers),
polymerization FPP blends of PP blocks with PE blocks, olefin block copolymers (OBC) resins of crystalline PE parts (blocks) and amorphous copolymerized PE parts (blocks) providing softness and high DSC melting temperature (plus or minus 120 °C), like INFUSE™ olefin block copolymers supplied by Dow, polyethylene plastomers (VLDPE) or LLDPE, i.e. PE obtained by copolymerization of ethylene with short-chain alpha- olefins (for example, 1 -butene, 1 -hexene and 1 -octene), with a density of less than 925 kg/m3 (ISO 1 183).
In another embodiment, the core layer may comprise an UV stabilized polyethylene terephthalate (PET), PET, polybutylene terephthalate PBT or PBT with an impact-modifying component, such as EP rubber, EPM rubber, EPDM rubber or SEBS.
In a further embodiment the core layer may comprise polypropylene homopolymers and/or polypropylene copolymers. The copolymers are further divided into block copolymers and random copolymers. Block copolymers of polypropylene have co-monomer units arranged in blocks (in a regular pattern) and contain for example between 5 wt% to 15 wt% ethylene. Random copolymers of polypropylene have co-monomer units arranged in irregular or random patterns along the
polypropylene chain. They are usually incorporated anywhere and contain between 1 wt% to 7wt % ethylene. Preferably the polypropylene layer comprises polypropylene impact copolymers. Typically, the impact polypropylene copolymers may be produced in a two-reactor system to yield a blend of polypropylene homopolymer with an ethylene-propylene rubber (EPR).
The rear layer of the backsheet may comprise a polyamide or fluoro- resin such as PTFE or PVDF. A polyamide layer can act as an oxygen barrier layer. The polyamide is preferably selected from the group consisting of polyamide 6;
polyamide 6,6; polyamide 4,6; polyamide 6,10; polyamide 6,12; polyamide 6,14;
polyamide 6,13; polyamide 6,15; polyamide 6,16; polyamide 1 1 ; polyamide 12;
polyamide 10; polyamide 9,12; polyamide 9,13; polyamide 9,14; polyamide 9,15;
polyamide 6,16; polyamide 10,10; polyamide 10,12; polyamide 10,13; polyamide 10,14; polyamide 12,10; polyamide 12,12; polyamide 12,13 or polyamide 12,14; polyamide 4,10, polyamide 5,10 or polyamide 6,10 or any mixtures thereof. Preferably polyamides are selected with limited moisture uptake such as polyamide 1 1 or 12. Low moisture uptake guarantees good electrical insulation also in high humidity environments. The PA layer can be rubber modified to further reduce the moisture uptake and increase the flexibility. The rear layer may also comprise an UV stabilized polyethylene
terephthalate (PET), polybutylene terephthalate PBT or PBT with an impact-modifying component, such as EP rubber, EPM rubber, EPDM rubber or SEBS.
The core and the rear layer typically comprise different polymeric materials. In one embodiment the core layer may comprise PET and the rear layer may comprise UV stabilized PET. In another embodiment the core layer may comprise polypropylene homopolymer and/or polypropylene copolymer and the rear layer may comprise a polyamide. In still another embodiment the core layer may comprise a polypropylene homopolymer and/or polypropylene copolymer and the rear layer may comprise a rubber modified PBT.
The thickness of the electro-electro-conductive back-sheet is preferably from 0.15 to 1 mm, more preferably from 0.15 to 0.8 mm, even more preferably from 0.15 to 0.75 mm.
The present invention also relates to a process for the manufacturing of the electroconductive backsheet comprising the following steps;
(a) the metal layer and the adhesive layer are laminated together into a metal-adhesive laminate and
(b) a polypropylene layer and optionally other polymeric layers are provided
(c) co-extrusion of the metal-adhesive laminate with the polypropylene layer and the optional other polymeric layers
(d) . optional subsequent patterning of the metal layer.
The laminating step a) is performed using a process well known to those skilled in the art by coating either or both the metal layer and polypropylene layer with the adhesive layer, drying of the adhesive layer and subsequent joining of the coated metal layer and/or polypropylene layer in for instance a roll-to-roll process. Curing of the adhesive layer is preferable performed at a temperature below 80 °C to avoid curling of the electro-conductive backsheet structure due to the differences in thermal expansion coefficient between the metal layer and the polypropylene layer.
In another embodiment, the process for the manufacturing of an electro-conductive backsheet comprises the following steps:
(a) the adhesive layer is laminated and/or extruded with a polypropylene layer and optionally other polymeric layers to provide a polypropylene and optionally other polymeric layers connected to an adhesive layer
(b) the metal layer is laminated on the laminated adhesive layer of (a)
(c) Optional subsequent patterning of the metal layer.
The present invention also relates to a photovoltaic module comprising the electro-conductive backsheet according to the present invention. A photovoltaic module (abbreviated PV module) comprises at least the following layers in order of position from the front sun-facing side to the back non-sun-facing side: a transparent pane (representing the front sheet), a front encapsulant layer, a solar cell layer, a back encapsulant layer, and the electro-conductive back sheet comprising a metal layer (1 ), an adhesive layer (2) to connect the metal layer to the backsheet wherein the backsheet comprises a polypropylene layer (3) with a melting point of at least 140 °C which connects to the metal layer via the adhesive layer. The backsheet may optionally comprise further polymeric layers such as a tie layer (4), a core layer (5) and/or a rear layer (6). A further description of these optional polymeric layers is given in the paragraphs above.
The front sheet is typically a glass plate or a rigid polymer sheet. The front and back encapsulant used in solar cell modules are designed to encapsulate and protect the fragile solar cells. The "front side" corresponds to a side of the photovoltaic cell irradiated with light, i.e. the light-receiving side, whereas the term "backside" corresponds to the reverse side of the light- receiving side of the photovoltaic cells.
Suitable encapsulants typically possess a combination of
characteristics such as high impact resistance, high penetration resistance, good ultraviolet (UV) light resistance, good long term thermal stability, adequate adhesion strength to glass and/or other rigid polymeric sheets, high moisture resistance, and good long-term weather ability. Currently, ethylene/vinyl acetate copolymers are the most widely used encapsulants.
Other suitable encapsulants include polyolefins, such as
polyethylenes homopolymers, polyethylene copolymers, but also polypropylenes such as polypropylene homopolymers and polypropylene copolymers; polyurethanes, polyvinyl butyrals, or silane comprising polymers and combinations of two or more thereof. Suitable ethylene copolymers include those comprising copolymerized units of ethylene and carboxylic acids such as (meth)acrylic acids (including esters thereof (i.e., acrylates) and salts thereof (i.e., ionomers)), and combinations of two or more thereof. When an polyethylene homopolymer is used, this may be a low density polyethylene, linear low density polyethylene, very low density polyethylene, ultra-low density polyethylene, medium density polyethylene, high density polyethylene, metallocene catalyzed polyethylene, and combinations of two or more of these polyethylenes.
The present invention further relates to a process for preparing such solar cell module, which process comprises (a) providing an assembly comprising all the polymer layers recited above and (b) laminating the assembly to form the solar cell module. The laminating step of the process may be conducted by subjecting the assembly to heat and optionally vacuum or pressure.
The invention is now demonstrated by means of a series of examples and comparative experiments.
FIGURES
Figure 1 shows the order of the metal layer (1 ), the adhesive layer (2) and the backsheet comprising the polypropylene layer (3). Figure 2 shows the order of the layers in a backsheet comprising the metal layer (1 ), the adhesive layer (2) and the backsheet comprising the polypropylene layer (3) and optionally more polymeric layers such as a tie layer (4), a core layer (5) and/or a rear layer(6).
Figure 3 shows the average pattern displacement which is measured by determining the difference in distances of the trenches of the pattern before and after lamination. EXAMPLES
I .Materials of the backsheet (TRIAL 1 )
Back-sheet 1 (BS1 ) having a thickness of 306 micron comprising of a layer facing the cells of PET from Mitsubishi Hostphan RN50-250 with a thickness of 250 microns and laminated with Adcote having a thickness of 6 microns to stabilized-PET from
Mitsubishi Hostaphan LHA50 with a thickness of 50 microns
Back-sheet 2 (BS2) B10 from DSM Sunshine having a thickness of 360 microns being a coextruded multilayer film comprising a layer facing the cells which layer comprises a LLDPE/EVA blend with a melting point of 105 °C. The other layers are based on a PP core layer and PA12 being the rear layer.
Back-sheet 3 (BS3) B10NE from DSM Sunshine having a thickness of 360 microns being a coextruded multilayer film comprising a layer facing the cells which layer comprising of polypropylene with a melting point of at least 140 °C and additives. The other layers are based on PP core layer and PA12 being the rear layer. Metal layer (trial 1 )
Copper (Cu, thickness 35 μηι): Circuit foil TZA
Physical vapor deposited copper (thickness 150 nm) on aluminium (Al/Cu PVD, thickness 67 μηη): Hanita
Cold spray deposited copper (thickness 5 μηη) on aluminium (Al/Cu CS, total thickness 50 μιτι): DSM
Electro-conductive back-sheets (ECB) (triaH ):
Electro-conductive back-sheets ECB1/ECB2 and ECB3 were produced by lamination. Lamination was performed using a process well known to those skilled in the art by coating both the metal layer and the cell facing layer of the back-sheet with the Adcote adhesive layer, drying of the adhesive layer for 15 minutes at room temperature and subsequent joining of the coated metal layer and/or cell-facing layer of the back-sheet polypropylene layer in for instance a roll-to-roll process. Curing of the adhesive layer was performed at room temperature for 3 weeks:
ECB1 : Cu-Adcote-BS1 ; ECB2: Cu-Adcote-BS2; ECB3: Cu-Adcote-BS3 Patterning of the electro-conductive back-sheets was performed by mechanical milling.
2x2 mini PV-modules were produced from the ECB1/ECB2/ECB3 through heat- lamination under vacuum at 150 °C for 15 minutes using EVA as encapsulant.
I. Results of the measurements (TRIAL 1 ) with the examples 1 -8 and comparative experiments l-IV is given below.
EXAMPLES 1 -2 and COMPARATIVE EXPERIMENT I
Damp-heat ageing: the elongation-at-break of the different electro-conductive back- sheets was determined before and after exposure for 1000 hrs to damp-heat (85 °C, 85% RH). The elongation-at-break was determined according to ISO 527.
Figure imgf000012_0001
Table A shows that the hydrolytic resistance of ECBs based on PP and PA12 is improved compared to PET-based ECBs
EXAMPLES 3-4 and COMPARATIVE EXPERIMENT II UV ageing: the elongation-at break of the different electro-conductive back-sheets was determined before and after exposure to 60 kWh UV in combination with 1000 hours damp-heat (DH, 85 °C, 85% RH). The elongation-at-break was determined according to ISO 527. ECB Elongation retention Elongation retention MD UV+DH [%] TD [%]
COMP. EXP. II ECB1 20 42
EXAMPLE 3 ECB2 75 69
EXAMPLE 4 ECB3 76 70
Table B shows that the UV resistance of ECBs based on PP and PA12 is improved compared to PET-based ECBs.
EXAMPLES 5-6 and COMPARATIVE EXPERIMENT III
Peel strength: peel strength between back-encapsulant and a mechanically milled area of the electro-conductive back-sheet was determined according to DIN EN 28510-1 .
Figure imgf000013_0001
EXAMPLES 7-8 and COMPARATIVE EXPERIMENT IV
Breakdown voltage is determined: in the middle of a 15x15cm sample an area of 10x10cm was mechanically milled. The breakdown voltage of the milled area was determined according to IEC 60243-1 and compared with the breakdown voltage of the initial back-sheet upon which the ECB was based. Comparison Breakdown voltage difference BS and milled area ECB [kV]
COMP. EXP IV BS1 -ECB1 19-16
EXAMPLE 7 BS2-ECB2 23-22
EXAMPLE 8 BS3-ECB3 24-24
Table C shows that there is no drop in breakdown voltage after milling for the electro- conductive back-sheet having the polypropylene layer facing the cells especially compared to the other ECBs. Also, the initial breakdown voltage is higher allowing higher system voltages.
II. Materials of the backsheet (TRIAL 2)
PE layer comprises 70 parts of LLDPE, 20 parts of EVA, 10 parts of Ti02 and 0.5 parts of an ultraviolet stabilizer, blended. Thickness of the PE layer is 50 μηι PP layer comprises 90 parts of copolymerized PP, 10 parts of Ti02 and 0.3 parts of Irganox 1010, blended. Thickness of the PP layer is 250 μηι
PA12 layer comprises 100 parts of PA12, 0.5 parts of Tinuvin770, 0.3 parts of Irganox B225 and 10 parts of Ti02, blended. Thickness of the PA12 layer is 25 μηι
Tie-layer between the PP and PE, between the PP and PA12 consists of 100 parts of maleic anhydride grafted polypropylene, thickness of the tie layer is 25 μηι
Metal layer (Trial 2)
Copper (Cu, thickness 35 μηι): Circuit foil TZA
Physical vapor deposited copper (thickness 150 nm) on aluminium (Al/Cu PVD, thickness 67 μηι): Hanita Cold spray deposited copper (thickness 5 μηι) on aluminium (Al/Cu CS, total thickness 50 μπι): DSM Method for manufacturing an electro-conductive backsheet (Trial 2) comprises the following steps:
A backsheet is prepared by a multilayer co-extrusion process whereby the pellets of the respective layers are added to a twin-screw extruder, melt-extruded at high temperature, and pushed through adapters through and a die, cooled by a cooling roller and shaped to manufacture the multi-layer back sheet. Subsequently the metal layer is coated using a K Control Benchtop Laboratory Laminator with 25-micron Adcote A3305 polyester adhesive in combination with CR857 isocyanate co-reactant (both available from Dow) resulting in an Adcote-coated metal substrate. The coating is left to dry for 15 minutes. Backsheets prepared by co-extrusion and a reference PVF- PET-PVF Krempel Akasol® PTL 3 HR 1000V backsheet are corona treated on the side to be adhered to the Adcote-coated metal substrate using a Tigres 600W corona treater. The corona treated backsheets are joined with Adcote-side of the Adcote- coated metal substrate using a Rycobel Benchtop Laboratory Laminator.
Subsequently, the Adcotejs allowed to cure in an oven at 60 °C for 2 days. Finally, the electro-conductive backsheets are patterned using an Eurotron IBP machine such that the final patterned electro-conductive backsheet is obtained. Compositions of the different layers in the electro-conductive backsheets are given in tables 3-5.
Method for manufacturing a 2x2 back-contacted mini PV-module (Trial 2)
2x2 back-contacted mini PV-modules were made using an Eurotron Euromini laboratory tool through laminating for 15 minutes at 145°C a stack of Scheuten: Super white 4 mm thick glass, a STR Photocap 15580P/UF (not punched) front encapsulant, JA Solar back-contact cells, a STR Photocap 15580P/UF (punched) back-encapsulant with EMS DB1588-4 conductive adhesive in the punched holes for connecting the cells to the metal substrate of the electro-conductive backsheet. In some cases, to be able to measure the displacement of the patterned metal layer of the electro-conductive backsheet after the lamination process, the cells and conductive adhesive are omitted. Measurements (Trial 2)
• Peel strength is measured at 90 °C angle using a Zwick Z050 tensile tester at 100 mm/min between the metal layer and the backsheet, and between the backsheet and the EVA back-encapsulant.
• The average pattern displacement is measured by determining the difference in distances of the trenches of the pattern before and after lamination as indicated in Figure 2.
• Power output performance is measured on 2x2 mini-modules at 1000 hours damp heat ageing (85%RH; 85C) time. Current (I) and Voltage (V)
characteristics of the solar cells prior to module manufacturing are measured using SUNSIM flash-tester and the IV characteristics of the modules are measured using a Pasan III A flash-tester under standard testing conditions [1000W/m2, AM1 .5 spectrum].
FF [%] is a parameter which, in conjunction with Voc and Isc, determines the maximum power from a solar cell. The FF is defined as the ratio of the maximum power from the solar cell to the_product of Voc and Isc. The change in power and FF are determined as an average of 4 2x2 mini-modules of the same build.
II. Results of the measurements (trial 2) with the examples 9-17 and comparative experiments V-XIII is given in the below tables. Table 1 shows peel strength measurements. Table 2 shows the displacement measurements. Table 3 shows the results of the changes in power and FF of 2x2 mini-modules.
EXAMPLES 9-17 and COMPARATIVE EXPERIMENTS V-XIII
In table 1 the peel strength between the metal substrate and the backsheet, and between the backsheet and the encapsulant is given. The peel strength between the backsheet and the EVA represents the peel strength in the trenches of the pattern where the metal and (part of the) adhesive are removed by milling.
Table 1 EXAMPLES Electro-conductive Peel strength Peel strength back-sheet metal-backsheet encapsulant -
[N/cml backsheet
PVD=physical vapor
[N/cml deposition
Cs=cold spray
9 PA12-PP-CU >40 unpeelable 74
10 PA12-PP-AI/CU PVD 24 74
1 1 PA12-PP-AI/CU Cs 32 74
COMPARATIVE EXPERIMENTS
V PA12-PP-PE-CU 36 71
VI PA12-PP-PE-AI/Cu 18 71
PVD
VII PA12-PP-PE-AI/Cu Cs 25 71
Table 1 shows the improved adhesive properties between the metal layer and PP-layer compared to using a backsheet based on PE with a melting point well below 140 °C. Also, the adhesive properties between the back-encapsulant and the backsheet are improved compared to the adhesive properties using a backsheet with a PE layer having a melting point of well below 140 °.
EXAMPLES 4-6 and COMPARATIVE EXPERIMENTS IV-VI
In table 2 the average displacement of the pattern after lamination is given. Table 2
Figure imgf000018_0001
Table 2 shows that the processing stability during lamination improves when replacing PE with a melting point of well below 140 °C by a PP-layer having a melting point of well above 140 °C.
EXAMPLES 7-9 and COMPARATIVE EXPERIMENTS VII-IX
In table 3 the change in power output performance and change in FF are given after 1000 hours dampheat ageing of 2x2 mini-modules.
Table 3
Figure imgf000019_0001
Table 3 shows that the improved adhesive properties between the metal layer and polypropylene layer, and between the back-encapsulant and the adhesive layer in combination with the improved processing stability during lamination lead to improved insulating properties and hence improved electrical performance of the back-contacted PV-modules. This results in higher output of the modules over time and hence increased lifetime. It also shown that the electrical performance and hence lifetime is also better compared to a backsheet that is not based on PP.

Claims

CLAIMS I . Electro-conductive backsheet for back contacted photovoltaic modules comprising:
(1 ) a metal layer,
(2) an adhesive layer
(3) a backsheet
characterized in that the backsheet comprises a polypropylene layer with a melting point of at least 140 °C whereby the polypropylene layer connects to the metal layer via the adhesive layer.
2. Electro-conductive backsheet according to claim 1 wherein the polypropylene layer comprises a polypropylene copolymer.
3. Electro-conductive backsheet according to any one of the claims 1 -2 wherein the polypropylene layer has a thickness equal or below 75 microns.
4. Electro-conductive backsheet according to any one of the claims 1 -3 wherein
polypropylene layer comprises additives chosen from the group consisting of fillers, stabilizers, colorants and/or pigments.
5. Electro-conductive backsheet according to claim 4 wherein the fillers are chosen from the group consisting of ΤΊ02, ZnO or ZnS.
6. Electro-conductive backsheet according to any one of the claims 1 -5 wherein the metal layer is a copper layer, aluminum layer or copper-coated aluminum layer.
7. Electro-conductive backsheet according to any one of the claims 1 -6 wherein the metal layer is patterned.
8. Electro-conductive backsheet according to any one of the claims 1 -7 wherein the adhesive layer comprises a polyurethane that is cured from a solvent-based polyester resin with an isocyanate crosslinking agent
9. Electro-conductive backsheet according to any one of the claims 1 -8 wherein the backsheet further comprises optional a tie layer, a core layer and/or a rear layer.
10. Electro-conductive backsheet according to any one of the claims 1 -9 wherein the backsheet further comprises a tie layer comprising a maleic anhydride grafted polyolefine.
I I . Electro-conductive backsheet according to claim 10 wherein the backsheet further comprises a core layer comprising a polypropylene homopolymer, a polypropylene copolymer, PET or PBT.
12. Electro-conductive backsheet according to claim 10 or 1 1 wherein the backsheet further comprises a rear layer comprising a polyamide or UV stabilized PET or PBT.
13. Process for the manufacturing of an electro-conductive backsheet according to any one of the claims 1 -12 wherein
(a) the metal layer and the adhesive layer are laminated into a metal-adhesive laminate and
(b) a polypropylene layer and optionally other polymeric layers are provided
(c) co-extrusion of the metal-adhesive laminate with the polypropylene layer and the optional other polymeric layers
(d). Optional subsequent patterning of the metal layer.
14. Process for the manufacturing of an electro-conductive backsheet according to any one of the claims 1 -12 wherein
(a) the adhesive layer is laminated and/or extruded with the polypropylene layer and optionally other polymeric layers
(c) the metal layer is laminated on the laminated adhesive layer of (a)
(d) . Optional subsequent patterning of the metal layer.
15. Photovoltaic module comprising the electro-conductive backsheet according to anyone of claims 1 -12.
PCT/EP2018/078273 2017-10-16 2018-10-16 Electro-conductive backsheet for solar cell modules WO2019076913A1 (en)

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
EP17196597 2017-10-16
EP17196597.3 2017-10-16
EP17201085.2 2017-11-10
EP17201085 2017-11-10

Publications (1)

Publication Number Publication Date
WO2019076913A1 true WO2019076913A1 (en) 2019-04-25

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Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2277694A1 (en) * 2009-07-23 2011-01-26 RENOLIT Belgium N.V. Photovoltaic modules using an adhesive integrated heat resistant multi-layer backsheet
US20130109125A1 (en) * 2011-10-31 2013-05-02 E I Du Pont De Nemours And Company Integrated back-sheet for back contact photovoltaic module
WO2013116876A2 (en) * 2012-02-03 2013-08-08 Avery Dennison Corporation Sheet assembly with aluminum based electrodes
EP2720519A1 (en) 2011-06-06 2014-04-16 Toppan Printing Co., Ltd. Metal foil patterned-laminate, metal foil laminate, metal foil laminate substrate, solar cell module and manufacturing method for metal foil patterned-laminate
US20140190545A1 (en) * 2013-01-10 2014-07-10 E I Du Pont De Nemours And Company Integrated back-sheet assembly for photovoltaic module
WO2015139284A1 (en) * 2014-03-21 2015-09-24 Dupont (China) Research & Development And Management Co., Ltd. Integrated back-sheets for back-contact solar cell modules

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2277694A1 (en) * 2009-07-23 2011-01-26 RENOLIT Belgium N.V. Photovoltaic modules using an adhesive integrated heat resistant multi-layer backsheet
EP2720519A1 (en) 2011-06-06 2014-04-16 Toppan Printing Co., Ltd. Metal foil patterned-laminate, metal foil laminate, metal foil laminate substrate, solar cell module and manufacturing method for metal foil patterned-laminate
US20130109125A1 (en) * 2011-10-31 2013-05-02 E I Du Pont De Nemours And Company Integrated back-sheet for back contact photovoltaic module
WO2013116876A2 (en) * 2012-02-03 2013-08-08 Avery Dennison Corporation Sheet assembly with aluminum based electrodes
US20140190545A1 (en) * 2013-01-10 2014-07-10 E I Du Pont De Nemours And Company Integrated back-sheet assembly for photovoltaic module
WO2015139284A1 (en) * 2014-03-21 2015-09-24 Dupont (China) Research & Development And Management Co., Ltd. Integrated back-sheets for back-contact solar cell modules

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