WO2012167705A1 - Solar cell backsheet with improved adhesion to encapsulant - Google Patents

Solar cell backsheet with improved adhesion to encapsulant Download PDF

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Publication number
WO2012167705A1
WO2012167705A1 PCT/CN2012/076220 CN2012076220W WO2012167705A1 WO 2012167705 A1 WO2012167705 A1 WO 2012167705A1 CN 2012076220 W CN2012076220 W CN 2012076220W WO 2012167705 A1 WO2012167705 A1 WO 2012167705A1
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WO
WIPO (PCT)
Prior art keywords
sheet
film
heat
fluoropolymer film
fluoropolymer
Prior art date
Application number
PCT/CN2012/076220
Other languages
French (fr)
Inventor
Zelin LIU
Qiuju Wu
Original Assignee
E.I. Du Pont De Nemours And Company
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
Application filed by E.I. Du Pont De Nemours And Company filed Critical E.I. Du Pont De Nemours And Company
Publication of WO2012167705A1 publication Critical patent/WO2012167705A1/en

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B37/00—Methods or apparatus for laminating, e.g. by curing or by ultrasonic bonding
    • B32B37/10—Methods or apparatus for laminating, e.g. by curing or by ultrasonic bonding characterised by the pressing technique, e.g. using action of vacuum or fluid pressure
    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C43/00—Compression moulding, i.e. applying external pressure to flow the moulding material; Apparatus therefor
    • B29C43/003—Compression moulding, i.e. applying external pressure to flow the moulding material; Apparatus therefor characterised by the choice of material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2309/00—Parameters for the laminating or treatment process; Apparatus details
    • B32B2309/02—Temperature
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2309/00—Parameters for the laminating or treatment process; Apparatus details
    • B32B2309/14—Velocity, e.g. feed speeds
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2310/00—Treatment by energy or chemical effects
    • B32B2310/08—Treatment by energy or chemical effects by wave energy or particle radiation
    • B32B2310/0806—Treatment by energy or chemical effects by wave energy or particle radiation using electromagnetic radiation
    • B32B2310/0825—Treatment by energy or chemical effects by wave energy or particle radiation using electromagnetic radiation using IR radiation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B37/00—Methods or apparatus for laminating, e.g. by curing or by ultrasonic bonding
    • B32B37/06—Methods or apparatus for laminating, e.g. by curing or by ultrasonic bonding characterised by the heating method
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B38/00—Ancillary operations in connection with laminating processes
    • B32B38/0036—Heat treatment
    • 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 disclosure herein is a fluoropolymerfilm or sheet with improved adhesion to other polymeric material and solar cell modules comprising the same.
  • solar cell modules electrically interconnected solar cells are often encapsulated by front and back encapsulant materials and the encapsulated solar cells are then sandwiched by a transparentfrontsheet and a backsheet.
  • the backsheets of the solar cell modules are used as supports and barriers from the environment.
  • those comprising fluoropolymers such as those having a multi-layer structure of polyvinyl fluoride/polyethylene terephthalate /polyvinyl fluoride (PVF/PET/PVF) have been used widely due to their superior weatherability,mechanical, electrical and barrier properties.
  • fluoropolymer containing backsheets one drawback of such fluoropolymer containing backsheets is that the bonding between the fluoropolymer film and the encapsulant material (e.g., ethyl ene/vinyl acetate copolymer (EVA)) may deteriorate over time and therefore cause de-lamination of the solar cell modules.
  • the encapsulant material e.g., ethyl ene/vinyl acetate copolymer (EVA)
  • EVA ethyl ene/vinyl acetate copolymer
  • the purpose of the present disclosure is to provide a method for obtaining a heat pressed fluoropolymer film or sheet that has improved bonding strength to a polyolefin film, the method comprising, (i) providing an oriented fluoropolymer film or sheet that consists essentially of fluoropolymer; and (ii) heat pressing the oriented fluoropolymer film or sheet by a heat press means, wherein the heat press means is set at such conditions that the oriented fluoropolymer film or sheet receives a pressure of 0.01 -50 Kgf/cm 2 and a heat of 150°C-260°C.
  • the heat press means used in step (ii) is set at such conditions that the oriented fluoropolymer film or sheet receives a pressure of 0.1 -50 Kgf/cm 2 , or preferably 0.5-50 Kgf/cm 2 , or more preferably 0.5-30 Kgf/cm 2 and a heat of 150°C-245°C, or preferably 160°C-220°C, or more preferably 160°C-200°C.
  • the heat press means used in step (ii) is a pair of hot flat plates and the oriented fluoropolymer film or sheet is heat pressed between the hot flat plates for 0.1 -30 sec, or preferably 0.5-30 sec, or more preferably 0.5-20 sec.
  • (ii) comprises at least one pair of heated nip rolls and the oriented fluoropolymer film or sheet is passed through the at least one pair of heated nip rolls that are set at a line speed of 0.01 -100 m/min, or preferably 0.1 -50 m/min, or more preferably 0.5-30 m/min.
  • the oriented fluoropolymer film or sheet is pre-heated to a temperature of 150°C-260°C, or preferably 150°C-245°C, or more preferably 160°C-220°C, or yet more preferably 160°C-200°C.
  • the oriented fluoropolymer film or sheet may bepre-heated by infra-red heat, air heat, flame heat, electron beam, or laser; or preferably the oriented fluoropolymer film or sheet is heated by an infra-red oven.
  • the step (ii) of heat pressing is repeated 2-50 times, or preferably 2-10 times.
  • the fluoropolymer is derived from fluoromonomersselected from the group consisting of vinyl fluorides, vinylidene fluorides, tetrafluoroethylenes, hexafluoropropylenes, fluorinated ethylene
  • the fluoropolymer may be selected from the group consisting of polyvinyl fluorides, polyvinylidene fluorides, and combinations thereof; or preferably from polyvinyl fluorides.
  • a heat pressed fluoropolymer film or sheet that is prepared by the any of the methods described above.
  • the heat pressed fluoropolymer film or sheet is laminated to a polyolefin film or sheet, and the bonding strength between the heat pressed fluoropolymer film or sheet and the polyolefin film or sheet, measured according to ASTM D903-38, is at least 17 N/cm, or preferably at least 20 N/cm, or more preferably at least 40 N/cm.
  • the polyolefin film or sheet may comprise a polyolefinic composition, wherein the polyolefinic composition comprises a material selected from the group consisting of ethylene/vinyl acetate copolymers, ionomers, polyethylenes,
  • ethylene/acrylate ester copolymers ethylene/acrylate ester copolymers, acid copolymers, and combinations of two or more thereof; or preferably from ethylene/vinyl acetate copolymers.
  • a multi-layer fluoropolymer film or sheet comprising at least one layer of the heat pressed fluoropolymer film or sheet described above.
  • the multi-layer fluoropolymer film or sheet may be in the form of a bi-layer film or sheet, and wherein the bi-layer film or sheet consists essentially of one layer of the heat pressed fluoropolymer film or sheet and one layer a polyester film or sheet that is laminated to the heat pressed fluoropolymer film or layer.
  • the multi-layer fluoropolymer film or sheet may be in the form of a tri-layer film or sheet, and wherein the tri-layer film or sheet consists essentially of two layers of the heat pressed fluoropolymer film or sheet and one layer of a polyester film or sheet that is laminated between the two heat pressed fluoropolymer film or sheet layers.
  • the polyester film or sheet may consist essentially of polyester selected from the group consisting of polyethylene terephthalates, polybutylene terephthalates, polytrimethylene terephthalates, polyethylene naphthalates, and combinations of two or more thereof.
  • a solar cell module comprising one or a plurality of solar cells, a back encapsulant sheet laminated to a back side of the solar cells, and a backsheet laminated to a back side of the back encapsulant sheet, wherein the back encapsulant sheet comprises a polyolefin, and wherein the backsheet comprises at least one layer of any of the heat pressed fluoropolymer films or sheets described above.
  • a solar cell module comprising one or a plurality of solar cells, a back encapsulant sheet laminated to a back side of the solar cells, and a backsheet laminated to a back side of the back encapsulant sheet, wherein the back encapsulant sheet comprises a polyolefin, and wherein the backsheet comprises any of the multi-layer fluoropolymer films or sheetsdescribed above.
  • the polyolefin comprising the back encapsulant sheet is selected from the group consisting of ethylene/vinyl acetate copolymers, ionomers, polyethylenes,
  • ethylene/acrylate ester copolymers ethylene/acrylate ester copolymers, acid copolymers, and combinations of two or more thereof, or preferably the polyolefin comprised in the back encapsulant sheet is selected from ethylene/vinyl acetate copolymers.
  • the range includes any value that is within the two particular end points and any value that is equal to or about equal to any of the two end points.
  • aheat pressed fluoropolymer filmor sheet Disclosed herein is aheat pressed fluoropolymer filmor sheet.
  • film and sheet are used interchangeably herein to refer to a continuous thin flat structure with a uniform thickness.
  • a sheet may have a thickness greater than about 100 ⁇ while a film may have a thickness of about 100 ⁇ or less.
  • the heat pressed fluoropolymer film or sheet is obtained by:(a)providing an oriented fluoropolymer film or sheetcomprising or consisting essentially of a fluoropolymer; and (b)heat pressing the oriented fluoropolymer film or sheet by a heat press means, wherein the heat press means is set at such conditions that the oriented fluoropolymer film or sheet receives a pressure of about 0.01 -50 kilograms-force per square centimeter (Kgf/cm 2 )and aheat of about 150°C-260°C.
  • a film or sheet “comprises or consists essentially of (a particular polymer)"
  • the film or sheet is made from (i) a material comprising the particular polymer and other components or (ii) a material consisting of the particular polymer and optionally certain other components, provided that the inclusion of the certain other components do not negatively affect the mechanical, physical, and adhesion properties of the film or sheet.
  • oriented refers to an orientation process, under which a polymeric film or sheet is uni-axially or bi-axially stretched in transverse and/or machine directions to achieve a combination of mechanical and physical properties.
  • Stretching apparatus and processes to obtain uni-axially or bi-axially oriented filmsor sheets are known in the art and may be adapted by those skilled in the art to produce the filmsor sheets disclosed herein. Examples of such apparatus and processes include, for example, those disclosed in U.S. PatentNos. 3,278,663; 3,337,665; 3,456,044; 4,590, 106; 4,760,1 16; 4,769,421 ; 4,797,235; and 4,886,634.
  • the oriented fluoropolymer films or sheets used herein may comprise or consist essentially of a fluoropolymer derived from fluoromonomersselected fromvinyl fluorides (VF), vinylidene fluorides (VDF), tetrafluoroethylenes (TFE), hexafluoropropylenes (HFP),fluorinated ethylene propylenes (FEP and EFEP), perfluoroalkoxys (PFA), chlorotrifluoroethlyenes (CTFE), and combinations of two or more thereof.
  • fluoropolymer derived from fluoromonomersselected fromvinyl fluorides (VF), vinylidene fluorides (VDF), tetrafluoroethylenes (TFE), hexafluoropropylenes (HFP),fluorinated ethylene propylenes (FEP and EFEP), perfluoroalkoxys (PFA), chlorotrifluoroethlyenes
  • fluoropolymers used herein include, without limitation, polyvinyl fluorides (PVF), polyvinylidene fluorides (PVDF), ethylene chlorotrifluoroethlyene copolymers (ECTFE),polychlorotrifluoroethylene (PCTFE), ethylene tetrafluoroethylene copolymers (ETFE), and combinations of two or more thereof (such as THV, a combination of TFE, HFP and VDF).
  • fluoropolymers or fluoromonomers may be copolymerized or blended with non- fluoropolymers to form oriented fluoropolymer films or sheets (such as HTE, a combination of HFP, TFE and ethylene).
  • oriented fluoropolymer films or sheets used herein are oriented PVF films or sheets comprising or consisting essentially of PVF, which is a thermoplastic fluoropolymer with repeating units of -(CH 2 CHF)n-.
  • PVF may be prepared by any suitable process, such as those disclosed in U.S. Patent No.
  • PVF has insufficient thermal stability for injection molding and is thus usually made into films or sheets via a solvent extrusion or casting process.
  • the oriented PVFfilm or sheet may be prepared byany suitable process, such as casting or solvent assisted extrusion.
  • U.S. Patent No. 2,953,818 discloses an extrusion process for the
  • the oriented fluoropolymer films or sheets used herein may also include those that have undergone various surface treatments to improve their bonding properties to other films or sheets.
  • Exemplary surface treatments include, without limitation, chemical treatment (see e.g., U.S. Patent No. 3,122,445), flame treatment (see e.g., U.S. Patent No.
  • the oriented PVFfilms or sheets used herein may also be obtained
  • suitable oriented PVF films or sheets may be any suitable oriented PVF films or sheets.
  • suitable oriented PVF films or sheets may be any suitable oriented PVF films or sheets.
  • the oriented fluoropolymer films or sheets used herein are oriented PVDF films or sheets comprising or consisting essentially of PVDF, which is a thermoplastic fluoropolymer with repeating units of -(CH 2 CF 2 ) n --
  • oriented PVDF films or sheets include, without limitation, KynarTM PVDF films from Arkema Inc. (U.S.A.) and Denka DX films from Denka Group (Japan).
  • the heat pressing process disclosed herein includessetting the heat press means at such conditions that the orientedfluoropolymer film or sheet receives a pressure of about 0.01 -50 Kgf/cm 2 , or about 0.1 -50 Kgf/cm 2 , or about 0.5-50 Kgf/cm 2 , or about 0.5-30 Kgf/cm 2 , and a heat of about 150°C-260°C, or about 150°C-245°C, or about 160°C-220°C, or about 160°C-200°C.
  • "receives a pressure of” means that the film or sheet is subjected to an average pressure of a certain force per unit area over the heat pressed portion of the film or sheet.
  • heat press means that as the film or sheet is contacted with a pressing surface that is heated to a certain temperature, the pressing surface applies the heat to the heat pressed portion of the film or sheet.
  • Any suitable heat press means may be used herein, which may include, without limitation, nip rolls, calendar rolls, flat bed laminators andhot flat plate press machines.
  • two or more rolls may be used in a horizontal or vertical configuration to heat press a multi-layer fluoropolymer film.
  • a three-roll extrusion coating line model number KXE1222, Davis- Standard, U.S.A.
  • a multi-layer fluoropolymer film or sheet may beunwound and passed between an upstream roll (steel) and acentralroll (rubber) under heat and pressure, and then cooled by a downstream roll (PTFE sleeved) before being wound up.
  • the roll temperatures, the line speed, and the nip pressure i.e., the pressure imposed on the films by the upstream and central rolls
  • a film or sheet may receive a heat that is the surface temperature of the heated rolls (e.g., the upstream steel roll and central rubber roll).
  • the surface temperatures of the heated rolls may be the same or different.
  • the upstream steel roll may be at a higher temperature than the central rubber roll.
  • the film or sheet When the film or sheet is pressed between two rolls having different surface temperatures, the film or sheet receives a heat that is the combined average temperature of the two rolls.
  • multiple nip rolls may be used to further control the heat pressing of the multi-layer fluoropolymer film.
  • the oriented fluoropolymer films or sheets are kept under pressure for about 0.1 -30 sec, or about 0.5-30 sec, or about 0.5-20 sec. In those embodiments wherein heated nip rolls or calendar rolls are used, it is preferred that the line speed of the rolls are kept at about 0.01 -100 m/min, or about 0.1-50 m/min, or about 0.5-30 m/min.
  • the heat pressing process may further comprise a pre-heating step wherein the orientedfluoropolymer film or sheet is pre-heated to a temperature of about 150°C-260°C by a heating source prior to the heat pressing step.
  • the heating source used herein may include, without limitation, infra-red (IR) heat (e.g., an IR oven), air heat, flame heat, electron beam and laser.
  • IR infra-red
  • the oriented fluoropolymer film or sheet may be heat pressed multiple times (e.g, about 2-50 times, or about 2-10 times).
  • the heat pressing process may include pressing the oriented fluoropolymer film or sheet between hot flat plates multiple times (such as, e.g., about 2-50 times, or about 2-10 times), wherein the film or sheet is cooled to room temperature between each pressing.
  • the oriented fluoropolymer film or sheet may be passed between heated nip rolls or calendar rolls multiple times, wherein the film or sheet is preferred to be cooled to room
  • the orientedfluoropolymer film or sheet is first pre-heated (e.g., by an IR oven) to a temperature of about 150°C- 260°C and then placed and pressed betweena pair of hot flat plates for about 0.1-30 sec, wherein the pair of hot flat plates is set at such conditions that the film or sheet receives a pressure of about 0.05-50 Kgf/cm 2 and a heat of about 150°C-260°C.
  • the oriented fluoropolymer film or sheet is first pre-heated (e.g., by an I R oven) to a temperature of about 150°C-260°C and then passed between at least one pair of heated nip rolls, wherein the at least one pair of heated nip rollsis set at a line speed of about 0.01 -100 m/min and other conditions such that the film or sheet receives a pressure of about 0.05-50 Kgf/cm 2 and a heat of about 150°C-260°C.
  • the oriented fluoropolymer film or sheet is passed between a pair of heated nip rolls multiple times(e.g., about 2-10 times), wherein the pair of heated nip rollsis set at a line speed of about 0.01 -100 m/min and other conditions such that the film or sheet receives a pressure of about 0.05-50 Kgf/cm 2 and a heat of about 150°C-260°C during each pass, and wherein the film or sheet is cooled to room temperature between each pass.
  • a pair of heated nip rolls multiple times(e.g., about 2-10 times)
  • the pair of heated nip rolls is set at a line speed of about 0.01 -100 m/min and other conditions such that the film or sheet receives a pressure of about 0.05-50 Kgf/cm 2 and a heat of about 150°C-260°C during each pass, and wherein the film or sheet is cooled to room temperature between each pass.
  • the oriented fluoropolymer film or sheet is passed between
  • each pair of the heated nip rolls is set at a line speed of about 0.01 -100 m/min and other conditions such that the film or sheet receives a pressure of about 0.05-50 Kgf/cm 2 and a heat of about 150°C-260°C, and wherein the multiple pairs of heated nip rolls are spaced in such a way that the film or sheet can be cooled to room temperature before being passed between the next pair of the heated nip rolls.
  • the final thickness of the heat pressed fluoropolymer film or sheet disclosed herein may have a total thickness of about 5-100 ⁇ , or about 10-50 ⁇ , or about 20- 40 ⁇ .
  • the bonding strength between the heat pressed fluoropolymer film or sheet and the EVA sheet was very much improved compared to the bonding strength between the oriented fluoropolymer film or sheetthat was not heat pressed and an EVA sheet.
  • EVA ethylene/vinyl acetate copolymer
  • the bonding strength between the heat pressed fluoropolymerfilm or sheet and the polyolefin film or sheet may reach a level of at least about 17N/cm, or at least about 20 N/cm, or at least about 40N/cm.
  • a laminated multi-layer fluoropolymer film or sheet comprising at least one layer of the heat pressed fluoropolymer film or sheet disclosed hereabove and one or more other additional layers.
  • Such one or more other additional layers may be formed of any suitable films or sheets, such as metal foils or polymeric films or sheets.
  • the polymeric films or sheets useful herein may be formed of any suitable polymeric material other than fluoropolymers.
  • the polymeric film or sheet used herein is a polyester film or sheet (a) comprising or consisting essentially of a polyester,or (b)comprising or consisting essentially of a polyesterselected from polyethylene terephthalates (PET), polybutylene terephthalates (PBT), polytri methylene terephthalates (PTT), polyethylene naphthalates (PEN), and combinations of two or more thereof; or (c) comprising or consisting essentially of a PET.
  • PET polyethylene terephthalates
  • PBT polybutylene terephthalates
  • PTT polytri methylene terephthalates
  • PEN polyethylene naphthalates
  • the laminated multi-layer fluoropolymer film or sheet is in the form of a laminated bi-layer fluoropolymer film or sheet consisting essentially of one layer of the heat pressed fluoropolymer film or sheet and one layer of a polyester film or sheet.
  • the laminated multi-layer fluoropolymer film or sheet is in the form of a laminated tri-layer fluoropolymer film or sheet consisting essentially of one layer of a polyester film or sheet laminated between two layers of the heat pressed fluoropolymer films or sheets.
  • a multilayer film or sheet is said to "consist essentially of”, it is meant that, in addition to the listed component layers, adhesives may or may not be also included in the subject multi-layer film or sheet to improve the bonding between the component layers.
  • the laminated multi-layer fluoropolymer films or sheets may be prepared by any suitable process, such as dry lamination or extrusion lamination.
  • adhesives may be included between any pair of adjacent layers of the multi-layer fluoropolymer films or sheets to improve the bonding strength therebetween.
  • Suitable adhesives may include, but are not limited to, polyurethanes, acrylics, epoxies, polyolefins and combinations of two or more thereof.
  • a solar cell module comprising one or a plurality of solar cells, a back encapsulant sheet laminated to a backside of the solar cells, and a backsheet laminated to a backside of the back encapsulant sheet, wherein the back encapsulant sheet comprises a polyolefin, and wherein the backsheet comprises at least one layer of the heat pressed fluoropolymer film or sheet disclosed above.
  • the solar cells used herein may be any photoelectric conversion device that can convert solar radiation to electrical energy.
  • solar cells may be formed of photoelectric conversion bodies with electrodes formed on both main surfaces thereof.
  • the photoelectric conversion bodies may be made of any suitable photoelectric conversion materials, such as, crystalline silicon (c-Si), amorphous silicon (a-Si), microcrystalline silicon ( ⁇ - ⁇ ), cadmium telluride (CdTe), copper indium selenide (CulnSe 2 or CIS), copper indium/gallium diselenide (Culn x Ga(i -X )Se2 or CIGS), light absorbing dyes, and organic semiconductors.
  • the front electrodes may be formed of conductive paste, such as silver paste, applied over the front surface of the photoelectric conversion body by any suitable printing process, such as screen printing or ink-jet printing.
  • the front conductive paste may comprise a plurality of parallel conductive fingers and one or more conductive bus bars perpendicular to and connecting the conductive fingers, while the back electrodes may be formed by printing metal paste over the entire back surface of the
  • Suitable metals forming the back electrodes include, but are not limited to, aluminum, copper, silver, gold, nickel, molybdenum, cadmium, and alloys thereof.
  • both electrodes of the photoelectric conversion body can be on the same surface thereof.
  • both the anode and cathode are located on the back surface of the photoelectric conversion body, forming a back contact solar cell.
  • each component layer within a solar cell module has a front surface (or side) and a back surface (or side).
  • the back encapsulant sheet that is laminated to the back side of the solar cells may comprise a polyolefin, including without limitation, ethylene/vinyl acetate copolymers (EVA), ionomers, polyethylenes, ethylene/acrylate ester copolymers (such as poly(ethylene-co-methyl acrylate) and poly(ethylene-co-butyl acrylate)), acid copolymers, and combinations of two or more thereof.
  • EVA ethylene/vinyl acetate copolymers
  • ionomers polyethylenes
  • ethylene/acrylate ester copolymers such as poly(ethylene-co-methyl acrylate) and poly(ethylene-co-butyl acrylate)
  • acid copolymers and combinations of two or more thereof.
  • the back encapsulant sheet comprises EVA.
  • EVA- based encapsulant sheets useful herein can be commercially obtained from
  • Exemplary ionomer-based encapsulant sheets include, without limitation, DuPontTM PV5300 series encapsulant sheets and DuPontTM PV5400 series encapsulant sheets from DuPont.
  • the backsheet of the solar cell module is formed ofthe heat pressed fluoropolymer film or sheet disclosed hereabove.
  • the backsheet is formed of thelaminated multi-layerfluoropolymer film or sheetdisclosed above, which comprises at least one layer of the heat pressed fluoropolymer film or sheet disclosed hereabove.
  • the solar cell modules disclosed herein may further comprise a transparent front encapsulant sheet laminated to a front surface of the solar cell(s), and a transparent frontsheet further laminated to a front surface of the front encapsulant sheet.
  • Suitable materials for the transparent front encapsulant sheet include without limitation, compositions comprisingEVA, ionomers, polyvinyl butyral) (PVB), polyurethane (PU), polyvinylchloride (PVC), polyethylenes, polyolefin block elastomers, ethyl ene/acryl ate ester copolymers (such as poly(ethylene-co-methyl acrylate) and poly(ethylene-co-butyl acrylate)), acid copolymers, silicone elastomers, epoxy resins, and the like.
  • PVB polyvinyl butyral
  • PU polyurethane
  • PVC polyvinylchloride
  • polyethylenes polyolefin block elastomers
  • ethyl ene/acryl ate ester copolymers such as poly(ethylene-co-methyl acrylate) and poly(ethylene-co-butyl acrylate)
  • acid copolymers such as poly(ethylene-co-methyl acryl
  • any suitable glass or plastic sheets can be used as the transparent front sheet.
  • suitable materials for the plastic frontsheet may include, without limitation, glass, polycarbonate, acrylics, polyacrylate, cyclic polyolefins, ethylene/norbornene polymers, metallocene-catalyzed polystyrene, polyamides, polyesters,
  • fluoropolymers and the like and combinations thereof.
  • any suitable lamination process may be used to produce the solar cell modules disclosed herein.
  • the process includes: (a) providing a plurality of electrically interconnected solar cells; (b) forming a pre-lamination assemblywherein the solar cells are laid over a back encapsulant sheet, which is further laid over a backsheet, wherein the backsheet comprises at least one layer of the heat pressed fluoropolymer film or sheet disclosed above; and (c) laminating the pre-lamination assemblyunder heat and pressure.
  • the process includes: (a) providing a plurality of electrically interconnected solar cells; (b) forming a pre-lamination assemblywherein the solar cells are sandwiched between a transparent front encapsulant sheet and a back encapsulant sheet, which is further sandwiched between a transparent frontsheet and a backsheet, wherein the backsheet comprises at least one layer of the heat pressed fluoropolymer film or sheet disclosed above; and (c) laminating the pre-lamination assemblyunder heat and pressure.
  • the module lamination process is performed using a ICOLAM 10/08 laminator purchased from Meier Solar Solutions GmbH (Germany) at about 135°C-150°C and about 1 atm for about 10-25 minutes.
  • PVF film Tedlar® PV2001 an orientedpolyvinyl fluoride film (38 ⁇ thick) obtained from DuPont;
  • EVA sheet RevaxTM 767 (3 ⁇ 43 ⁇ 4 767) ethylene/vinyl acetate sheet (500 ⁇ thick) obtained from RuiYangPhotovoltaic Material Co. Ltd. (China);
  • PET film corona treated (both sides) MelinexTM S polyethylene terephthalate film (250 ⁇ thick) obtained from DuPont Teijin Films (U.S.A.).
  • MelinexTM S polyethylene terephthalate film 250 ⁇ thick obtained from DuPont Teijin Films (U.S.A.). Comparative Examples CE1-CE2 and Examples E1 -E7:
  • CE1 a number of laminated sheets having a structure of "glass/EVA sheet/PVF film/PET film/PVF film" were prepared as follows. First, a layer of PET film was laminated betweentwo layers of PVF film, that were not heat
  • CE2 and E1 -E7 a number of laminated sheets having a structure of "glass/EVA sheet/heat pressed PVF film/PET film/heat pressed PVF film" were prepared following substantially the same method as those disclosed above in CE1 , with the exception that heat pressed PVF films were used in place of the non-heat pressed PVF films of CE1.
  • the various heat pressed PVF filmsused in each of CE2 and E1 -E7 were obtained by placing a layer of PVF film in an infra red (IR) oven (Model Number 10831010, purchased from Shanghai Yuejin Medical Instruments Factory, China) for a certain period of time, which was followed by passing the heated PVF films through a pair of heated nip rolls.
  • IR infra red
  • heat pressed PVF films had improved bonding strength with the EVA sheets (ranging from 29.8-76.2 N/cm in E1 -E7) compared to that of the untreated PVF films (10.1 N/cm in CE1 ).

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Abstract

Disclosed herein is heat pressed fluoropolymer film or sheetand methods for preparing the same, and which methods comprise the steps of (i) providing an oriented fluoropolymer film or sheet consisting essentially of a fluoropolymer; and (ii) heat pressing the oriented fluoropolymer film or sheet by a heat press means, wherein the heat press means is set at such conditions that the oriented fluoropolymer film or sheet receives a pressure of about 0.01 -50 Kgf/cm2 and a heat of about 160 °C-260°C. Further disclosed herein are solar cell modules comprisingbacksheets that comprises at least one layer of the heat pressed fluoropolymer films or sheets.

Description

SOLAR CELL BACKSHEET WITH IMPROVED
ADHESION TO ENCAPSULANT
Field of Disclosure
The disclosure herein is a fluoropolymerfilm or sheet with improved adhesion to other polymeric material and solar cell modules comprising the same.
Background
In solar cell modules, electrically interconnected solar cells are often encapsulated by front and back encapsulant materials and the encapsulated solar cells are then sandwiched by a transparentfrontsheet and a backsheet. The backsheets of the solar cell modules are used as supports and barriers from the environment. Among the prior art backsheets, those comprising fluoropolymers (such as those having a multi-layer structure of polyvinyl fluoride/polyethylene terephthalate /polyvinyl fluoride (PVF/PET/PVF)) have been used widely due to their superior weatherability,mechanical, electrical and barrier properties. However, one drawback of such fluoropolymer containing backsheets is that the bonding between the fluoropolymer film and the encapsulant material (e.g., ethyl ene/vinyl acetate copolymer (EVA)) may deteriorate over time and therefore cause de-lamination of the solar cell modules. Thus, there is still a need to develop fluoropolymer containing backsheets having good adhesion to the encapsulant material.
Summary
The purpose of the present disclosure is to provide a method for obtaining a heat pressed fluoropolymer film or sheet that has improved bonding strength to a polyolefin film, the method comprising, (i) providing an oriented fluoropolymer film or sheet that consists essentially of fluoropolymer; and (ii) heat pressing the oriented fluoropolymer film or sheet by a heat press means, wherein the heat press means is set at such conditions that the oriented fluoropolymer film or sheet receives a pressure of 0.01 -50 Kgf/cm2 and a heat of 150°C-260°C. In one embodiment of the method, the heat press means used in step (ii) is set at such conditions that the oriented fluoropolymer film or sheet receives a pressure of 0.1 -50 Kgf/cm2, or preferably 0.5-50 Kgf/cm2, or more preferably 0.5-30 Kgf/cm2 and a heat of 150°C-245°C, or preferably 160°C-220°C, or more preferably 160°C-200°C.
In a further embodiment of the method, the heat press means used in step (ii) is a pair of hot flat plates and the oriented fluoropolymer film or sheet is heat pressed between the hot flat plates for 0.1 -30 sec, or preferably 0.5-30 sec, or more preferably 0.5-20 sec.
In a yet further embodiment of the method, the heat press means used in step
(ii) comprises at least one pair of heated nip rolls and the oriented fluoropolymer film or sheet is passed through the at least one pair of heated nip rolls that are set at a line speed of 0.01 -100 m/min, or preferably 0.1 -50 m/min, or more preferably 0.5-30 m/min.
In a yet further embodiment of the method, wherein, prior to step (ii), the oriented fluoropolymer film or sheet is pre-heated to a temperature of 150°C-260°C, or preferably 150°C-245°C, or more preferably 160°C-220°C, or yet more preferably 160°C-200°C. And, the oriented fluoropolymer film or sheet may bepre-heated by infra-red heat, air heat, flame heat, electron beam, or laser; or preferably the oriented fluoropolymer film or sheet is heated by an infra-red oven.
In a yet further embodiment of the method, the step (ii) of heat pressing is repeated 2-50 times, or preferably 2-10 times.
In a yet further embodiment of the method, the fluoropolymer is derived from fluoromonomersselected from the group consisting of vinyl fluorides, vinylidene fluorides, tetrafluoroethylenes, hexafluoropropylenes, fluorinated ethylene
propylenes, perfluoroalkoxys, chlorotrifluoroethlyenes, and combinations of two or more thereof. Or, the fluoropolymer may be selected from the group consisting of polyvinyl fluorides, polyvinylidene fluorides, and combinations thereof; or preferably from polyvinyl fluorides.
Further provided herein is a heat pressed fluoropolymer film or sheet that is prepared by the any of the methods described above. In one embodiment of the heat pressed fluoropolymer film or sheet, the heat pressed fluoropolymer film or sheet is laminated to a polyolefin film or sheet, and the bonding strength between the heat pressed fluoropolymer film or sheet and the polyolefin film or sheet, measured according to ASTM D903-38, is at least 17 N/cm, or preferably at least 20 N/cm, or more preferably at least 40 N/cm. In such embodiments, the polyolefin film or sheet may comprise a polyolefinic composition, wherein the polyolefinic composition comprises a material selected from the group consisting of ethylene/vinyl acetate copolymers, ionomers, polyethylenes,
ethylene/acrylate ester copolymers, acid copolymers, and combinations of two or more thereof; or preferably from ethylene/vinyl acetate copolymers.
Yet further provided herein is a multi-layer fluoropolymer film or sheet comprising at least one layer of the heat pressed fluoropolymer film or sheet described above. The multi-layer fluoropolymer film or sheet may be in the form of a bi-layer film or sheet, and wherein the bi-layer film or sheet consists essentially of one layer of the heat pressed fluoropolymer film or sheet and one layer a polyester film or sheet that is laminated to the heat pressed fluoropolymer film or layer. Or, the multi-layer fluoropolymer film or sheet may be in the form of a tri-layer film or sheet, and wherein the tri-layer film or sheet consists essentially of two layers of the heat pressed fluoropolymer film or sheet and one layer of a polyester film or sheet that is laminated between the two heat pressed fluoropolymer film or sheet layers. In such embodiments, the polyester film or sheet may consist essentially of polyester selected from the group consisting of polyethylene terephthalates, polybutylene terephthalates, polytrimethylene terephthalates, polyethylene naphthalates, and combinations of two or more thereof.
Yet further provided herein is a solar cell module comprising one or a plurality of solar cells, a back encapsulant sheet laminated to a back side of the solar cells, and a backsheet laminated to a back side of the back encapsulant sheet, wherein the back encapsulant sheet comprises a polyolefin, and wherein the backsheet comprises at least one layer of any of the heat pressed fluoropolymer films or sheets described above. Yet further provided herein is a solar cell module comprising one or a plurality of solar cells, a back encapsulant sheet laminated to a back side of the solar cells, and a backsheet laminated to a back side of the back encapsulant sheet, wherein the back encapsulant sheet comprises a polyolefin, and wherein the backsheet comprises any of the multi-layer fluoropolymer films or sheetsdescribed above.
In certain embodiments of the solar cell modules described above, the polyolefin comprising the back encapsulant sheet is selected from the group consisting of ethylene/vinyl acetate copolymers, ionomers, polyethylenes,
ethylene/acrylate ester copolymers, acid copolymers, and combinations of two or more thereof, or preferably the polyolefin comprised in the back encapsulant sheet is selected from ethylene/vinyl acetate copolymers.
In accordance with the present disclosure, when a range is given with two particular end points, it is understood that the range includes any value that is within the two particular end points and any value that is equal to or about equal to any of the two end points.
Detailed Description
Disclosed herein is aheat pressed fluoropolymer filmor sheet. The terms "film" and "sheet" are used interchangeably herein to refer to a continuous thin flat structure with a uniform thickness. In general, a sheet may have a thickness greater than about 100 μηη while a film may have a thickness of about 100 μηη or less. In accordance with the present disclosure, the heat pressed fluoropolymer film or sheet is obtained by:(a)providing an oriented fluoropolymer film or sheetcomprising or consisting essentially of a fluoropolymer; and (b)heat pressing the oriented fluoropolymer film or sheet by a heat press means, wherein the heat press means is set at such conditions that the oriented fluoropolymer film or sheet receives a pressure of about 0.01 -50 kilograms-force per square centimeter (Kgf/cm2)and aheat of about 150°C-260°C.
When it is said that a film or sheet "comprises or consists essentially of (a particular polymer)", it is meant that the film or sheet is made from (i) a material comprising the particular polymer and other components or (ii) a material consisting of the particular polymer and optionally certain other components, provided that the inclusion of the certain other components do not negatively affect the mechanical, physical, and adhesion properties of the film or sheet.
The term "oriented", as used herein, refers to an orientation process, under which a polymeric film or sheet is uni-axially or bi-axially stretched in transverse and/or machine directions to achieve a combination of mechanical and physical properties. Stretching apparatus and processes to obtain uni-axially or bi-axially oriented filmsor sheets are known in the art and may be adapted by those skilled in the art to produce the filmsor sheets disclosed herein. Examples of such apparatus and processes include, for example, those disclosed in U.S. PatentNos. 3,278,663; 3,337,665; 3,456,044; 4,590, 106; 4,760,1 16; 4,769,421 ; 4,797,235; and 4,886,634.
The oriented fluoropolymer films or sheets used herein may comprise or consist essentially of a fluoropolymer derived from fluoromonomersselected fromvinyl fluorides (VF), vinylidene fluorides (VDF), tetrafluoroethylenes (TFE), hexafluoropropylenes (HFP),fluorinated ethylene propylenes (FEP and EFEP), perfluoroalkoxys (PFA), chlorotrifluoroethlyenes (CTFE), and combinations of two or more thereof. More specific exemplary fluoropolymers used herein include, without limitation, polyvinyl fluorides (PVF), polyvinylidene fluorides (PVDF), ethylene chlorotrifluoroethlyene copolymers (ECTFE),polychlorotrifluoroethylene (PCTFE), ethylene tetrafluoroethylene copolymers (ETFE), and combinations of two or more thereof (such as THV, a combination of TFE, HFP and VDF). In some embodiments, fluoropolymers or fluoromonomers may be copolymerized or blended with non- fluoropolymers to form oriented fluoropolymer films or sheets (such as HTE, a combination of HFP, TFE and ethylene).
In one embodiment, oriented fluoropolymer films or sheets used herein are oriented PVF films or sheets comprising or consisting essentially of PVF, which is a thermoplastic fluoropolymer with repeating units of -(CH2CHF)n-. PVF may be prepared by any suitable process, such as those disclosed in U.S. Patent No.
2,419,010. In general, PVF has insufficient thermal stability for injection molding and is thus usually made into films or sheets via a solvent extrusion or casting process. In accordance with the present disclosure, the oriented PVFfilm or sheet may be prepared byany suitable process, such as casting or solvent assisted extrusion. For example, U.S. Patent No. 2,953,818 discloses an extrusion process for the
preparation of orientablefilms from PVF and U.S. Patent No. 3,139,470 discloses a process for preparing oriented PVF films.
Also in accordance with the present disclosure, the oriented fluoropolymer films or sheets used herein may also include those that have undergone various surface treatments to improve their bonding properties to other films or sheets.
Exemplary surface treatments include, without limitation, chemical treatment (see e.g., U.S. Patent No. 3,122,445), flame treatment (see e.g., U.S. Patent No.
3,145,242), and electrical discharge treatment (see e.g., U.S. Patent No. 3,274,088).
The oriented PVFfilms or sheets used herein may also be obtained
commercially. For example, suitable oriented PVF films or sheets may be
purchased from E.I. du Pont de Nemours and Company (U.S.A.) (hereafter "DuPont") under the trade name Tedlar®.
In another embodiment, the oriented fluoropolymer films or sheets used herein are oriented PVDF films or sheets comprising or consisting essentially of PVDF, which is a thermoplastic fluoropolymer with repeating units of -(CH2CF2)n-- Commercially available oriented PVDF films or sheets, include, without limitation, Kynar™ PVDF films from Arkema Inc. (U.S.A.) and Denka DX films from Denka Group (Japan).
The heat pressing process disclosed herein includessetting the heat press means at such conditions that the orientedfluoropolymer film or sheet receives a pressure of about 0.01 -50 Kgf/cm2, or about 0.1 -50 Kgf/cm2, or about 0.5-50 Kgf/cm2, or about 0.5-30 Kgf/cm2, and a heat of about 150°C-260°C, or about 150°C-245°C, or about 160°C-220°C, or about 160°C-200°C. As used herein, "receives a pressure of" means that the film or sheet is subjected to an average pressure of a certain force per unit area over the heat pressed portion of the film or sheet. As used herein, "receives a heat of" means that as the film or sheet is contacted with a pressing surface that is heated to a certain temperature, the pressing surface applies the heat to the heat pressed portion of the film or sheet. Any suitable heat press means may be used herein, which may include, without limitation, nip rolls, calendar rolls, flat bed laminators andhot flat plate press machines. In one embodiment, two or more rolls may be used in a horizontal or vertical configuration to heat press a multi-layer fluoropolymer film. In a more specific embodiment, a three-roll extrusion coating line (model number KXE1222, Davis- Standard, U.S.A.) with a horizontal roll configuration may be used. A multi-layer fluoropolymer film or sheet may beunwound and passed between an upstream roll (steel) and acentralroll (rubber) under heat and pressure, and then cooled by a downstream roll (PTFE sleeved) before being wound up. The roll temperatures, the line speed, and the nip pressure (i.e., the pressure imposed on the films by the upstream and central rolls) can all be adjusted. A film or sheet may receive a heat that is the surface temperature of the heated rolls (e.g., the upstream steel roll and central rubber roll). The surface temperatures of the heated rolls may be the same or different. In one embodiment, the upstream steel roll may be at a higher temperature than the central rubber roll. When the film or sheet is pressed between two rolls having different surface temperatures, the film or sheet receives a heat that is the combined average temperature of the two rolls. In another embodiment, multiple nip rolls may be used to further control the heat pressing of the multi-layer fluoropolymer film.
In those embodiments wherein hot flat plates are used, it is preferred that the oriented fluoropolymer films or sheets are kept under pressure for about 0.1 -30 sec, or about 0.5-30 sec, or about 0.5-20 sec. In those embodiments wherein heated nip rolls or calendar rolls are used, it is preferred that the line speed of the rolls are kept at about 0.01 -100 m/min, or about 0.1-50 m/min, or about 0.5-30 m/min. Further, in certain embodiments, the heat pressing process may further comprise a pre-heating step wherein the orientedfluoropolymer film or sheet is pre-heated to a temperature of about 150°C-260°C by a heating source prior to the heat pressing step. The heating source used herein may include, without limitation, infra-red (IR) heat (e.g., an IR oven), air heat, flame heat, electron beam and laser. Alternatively, the oriented fluoropolymer film or sheet may be heat pressed multiple times (e.g, about 2-50 times, or about 2-10 times). For example, the heat pressing process may include pressing the oriented fluoropolymer film or sheet between hot flat plates multiple times (such as, e.g., about 2-50 times, or about 2-10 times), wherein the film or sheet is cooled to room temperature between each pressing. Also, the oriented fluoropolymer film or sheet may be passed between heated nip rolls or calendar rolls multiple times, wherein the film or sheet is preferred to be cooled to room
temperature between each pass. In one embodiment, the orientedfluoropolymer film or sheet is first pre-heated (e.g., by an IR oven) to a temperature of about 150°C- 260°C and then placed and pressed betweena pair of hot flat plates for about 0.1-30 sec, wherein the pair of hot flat plates is set at such conditions that the film or sheet receives a pressure of about 0.05-50 Kgf/cm2 and a heat of about 150°C-260°C.
In one embodiment, the oriented fluoropolymer film or sheet is first pre-heated (e.g., by an I R oven) to a temperature of about 150°C-260°C and then passed between at least one pair of heated nip rolls, wherein the at least one pair of heated nip rollsis set at a line speed of about 0.01 -100 m/min and other conditions such that the film or sheet receives a pressure of about 0.05-50 Kgf/cm2 and a heat of about 150°C-260°C. In a yet further embodiment, the oriented fluoropolymer film or sheet is passed between a pair of heated nip rolls multiple times(e.g., about 2-10 times), wherein the pair of heated nip rollsis set at a line speed of about 0.01 -100 m/min and other conditions such that the film or sheet receives a pressure of about 0.05-50 Kgf/cm2 and a heat of about 150°C-260°C during each pass, and wherein the film or sheet is cooled to room temperature between each pass. In a yet further
embodiment, the oriented fluoropolymer film or sheet is passed between
multiple(e.g., about 2-10) pairs of heated nip rolls, wherein each pair of the heated nip rollsis set at a line speed of about 0.01 -100 m/min and other conditions such that the film or sheet receives a pressure of about 0.05-50 Kgf/cm2 and a heat of about 150°C-260°C, and wherein the multiple pairs of heated nip rolls are spaced in such a way that the film or sheet can be cooled to room temperature before being passed between the next pair of the heated nip rolls. Those skilled in the art will understand that a variety of configurations may be used for controlling the heating, pressing and cooling of the multi-layer fluoropolymer film. The final thickness of the heat pressed fluoropolymer film or sheet disclosed herein may have a total thickness of about 5-100μηη, or about 10-50μηη, or about 20- 40 μηη.
As demonstrated by the examples provided herebelow, when the heat pressed fluoropolymer film or sheet disclosed herein was laminated to an
ethylene/vinyl acetate copolymer (EVA) sheet, the bonding strength between the heat pressed fluoropolymer film or sheet and the EVA sheet was very much improved compared to the bonding strength between the oriented fluoropolymer film or sheetthat was not heat pressed and an EVA sheet. For example, when a prior art oriented fluoropolymer film orsheet was laminated to an EVA sheet, its bonding strength to the EVA sheet was only 10.1 N/cm. However, when a heat pressed fluoropolymer film orsheet (as described above) was laminated to an EVA sheet in the same manner, itsbonding strength tothe EVA sheet could be increased to up to 76.2N/cm.
Therefore, in accordance with the present disclosure, when the heat pressed fluoropolymer film or sheet is laminated to a polyolefin film or sheet (e.g., an EVA film or sheet), the bonding strength between the heat pressedfluoropolymerfilm or sheet and the polyolefin film or sheet may reach a level of at least about 17N/cm, or at least about 20 N/cm, or at least about 40N/cm.
Further disclosed herein is a laminated multi-layer fluoropolymer film or sheet comprising at least one layer of the heat pressed fluoropolymer film or sheet disclosed hereabove and one or more other additional layers. Such one or more other additional layers may be formed of any suitable films or sheets, such as metal foils or polymeric films or sheets. The polymeric films or sheets useful herein may be formed of any suitable polymeric material other than fluoropolymers. In one embodiment, the polymeric film or sheet used herein is a polyester film or sheet (a) comprising or consisting essentially of a polyester,or (b)comprising or consisting essentially of a polyesterselected from polyethylene terephthalates (PET), polybutylene terephthalates (PBT), polytri methylene terephthalates (PTT), polyethylene naphthalates (PEN), and combinations of two or more thereof; or (c) comprising or consisting essentially of a PET. In one embodiment, the laminated multi-layer fluoropolymer film or sheet is in the form of a laminated bi-layer fluoropolymer film or sheet consisting essentially of one layer of the heat pressed fluoropolymer film or sheet and one layer of a polyester film or sheet. In a further embodiment, the laminated multi-layer fluoropolymer film or sheet is in the form of a laminated tri-layer fluoropolymer film or sheet consisting essentially of one layer of a polyester film or sheet laminated between two layers of the heat pressed fluoropolymer films or sheets.When a multilayer film or sheet is said to "consist essentially of", it is meant that, in addition to the listed component layers, adhesives may or may not be also included in the subject multi-layer film or sheet to improve the bonding between the component layers.
The laminated multi-layer fluoropolymer films or sheets may be prepared by any suitable process, such as dry lamination or extrusion lamination. In addition, adhesives may be included between any pair of adjacent layers of the multi-layer fluoropolymer films or sheets to improve the bonding strength therebetween.
Suitable adhesives may include, but are not limited to, polyurethanes, acrylics, epoxies, polyolefins and combinations of two or more thereof.
Further disclosed herein is a solar cell module comprising one or a plurality of solar cells, a back encapsulant sheet laminated to a backside of the solar cells, and a backsheet laminated to a backside of the back encapsulant sheet, wherein the back encapsulant sheet comprises a polyolefin, and wherein the backsheet comprises at least one layer of the heat pressed fluoropolymer film or sheet disclosed above.
The solar cells used herein may be any photoelectric conversion device that can convert solar radiation to electrical energy. In one embodiment, solar cells may be formed of photoelectric conversion bodies with electrodes formed on both main surfaces thereof. The photoelectric conversion bodies may be made of any suitable photoelectric conversion materials, such as, crystalline silicon (c-Si), amorphous silicon (a-Si), microcrystalline silicon (μο-βί), cadmium telluride (CdTe), copper indium selenide (CulnSe2 or CIS), copper indium/gallium diselenide (CulnxGa(i-X)Se2 or CIGS), light absorbing dyes, and organic semiconductors. The front electrodes may be formed of conductive paste, such as silver paste, applied over the front surface of the photoelectric conversion body by any suitable printing process, such as screen printing or ink-jet printing. The front conductive paste may comprise a plurality of parallel conductive fingers and one or more conductive bus bars perpendicular to and connecting the conductive fingers, while the back electrodes may be formed by printing metal paste over the entire back surface of the
photoelectric conversion body. Suitable metals forming the back electrodes include, but are not limited to, aluminum, copper, silver, gold, nickel, molybdenum, cadmium, and alloys thereof. In another embodiment, both electrodes of the photoelectric conversion body can be on the same surface thereof. In a particular embodiment, both the anode and cathode are located on the back surface of the photoelectric conversion body, forming a back contact solar cell.
When in use, the solar cells typically have a front (or top) surface facing towards the solar radiation and a back (or bottom) surface facing away from the solar radiation. Therefore, each component layer within a solar cell module has a front surface (or side) and a back surface (or side).
In accordance with the present disclosure, the back encapsulant sheet that is laminated to the back side of the solar cells may comprise a polyolefin, including without limitation, ethylene/vinyl acetate copolymers (EVA), ionomers, polyethylenes, ethylene/acrylate ester copolymers (such as poly(ethylene-co-methyl acrylate) and poly(ethylene-co-butyl acrylate)), acid copolymers, and combinations of two or more thereof. In one embodiment, the back encapsulant sheet comprises EVA. EVA- based encapsulant sheets useful herein can be commercially obtained from
Bridgestone (Japan) under the trade name EVASKY™;Sanvic Inc. (Japan) under the trade name Ultrapearl™; Bixby International Corp. (U.S.A.) under the trade name BixCure™; orRuiYang Photovoltaic Material Co. Ltd. (China) under the trade name Revax™. Exemplary ionomer-based encapsulant sheets include, without limitation, DuPont™ PV5300 series encapsulant sheets and DuPont™ PV5400 series encapsulant sheets from DuPont.
In one embodiment, the backsheet of the solar cell module is formed ofthe heat pressed fluoropolymer film or sheet disclosed hereabove. In a further embodiment, the backsheet is formed of thelaminated multi-layerfluoropolymer film or sheetdisclosed above, which comprises at least one layer of the heat pressed fluoropolymer film or sheet disclosed hereabove.
The solar cell modules disclosed herein may further comprise a transparent front encapsulant sheet laminated to a front surface of the solar cell(s), and a transparent frontsheet further laminated to a front surface of the front encapsulant sheet.
Suitable materials for the transparent front encapsulant sheet includewithout limitation, compositions comprisingEVA, ionomers, polyvinyl butyral) (PVB), polyurethane (PU), polyvinylchloride (PVC), polyethylenes, polyolefin block elastomers, ethyl ene/acryl ate ester copolymers (such as poly(ethylene-co-methyl acrylate) and poly(ethylene-co-butyl acrylate)), acid copolymers, silicone elastomers, epoxy resins, and the like.
Any suitable glass or plastic sheets can be used as the transparent front sheet. Suitable materials for the plastic frontsheet may include, without limitation, glass, polycarbonate, acrylics, polyacrylate, cyclic polyolefins, ethylene/norbornene polymers, metallocene-catalyzed polystyrene, polyamides, polyesters,
fluoropolymers and the like and combinations thereof.
Any suitable lamination process may be used to produce the solar cell modules disclosed herein. In one embodiment, the process includes: (a) providing a plurality of electrically interconnected solar cells; (b) forming a pre-lamination assemblywherein the solar cells are laid over a back encapsulant sheet, which is further laid over a backsheet, wherein the backsheet comprises at least one layer of the heat pressed fluoropolymer film or sheet disclosed above; and (c) laminating the pre-lamination assemblyunder heat and pressure.
In a further embodiment, the process includes: (a) providing a plurality of electrically interconnected solar cells; (b) forming a pre-lamination assemblywherein the solar cells are sandwiched between a transparent front encapsulant sheet and a back encapsulant sheet, which is further sandwiched between a transparent frontsheet and a backsheet, wherein the backsheet comprises at least one layer of the heat pressed fluoropolymer film or sheet disclosed above; and (c) laminating the pre-lamination assemblyunder heat and pressure. In one embodiment, the module lamination process is performed using a ICOLAM 10/08 laminator purchased from Meier Solar Solutions GmbH (Germany) at about 135°C-150°C and about 1 atm for about 10-25 minutes.
Examples
Materials Used:
• PVF film: Tedlar® PV2001 an orientedpolyvinyl fluoride film (38 μηη thick) obtained from DuPont;
• EVA sheet: Revax™ 767 (¾¾ 767) ethylene/vinyl acetate sheet (500 μηι thick) obtained from RuiYangPhotovoltaic Material Co. Ltd. (China);
PET film: corona treated (both sides) Melinex™ S polyethylene terephthalate film (250 μηη thick) obtained from DuPont Teijin Films (U.S.A.). Comparative Examples CE1-CE2 and Examples E1 -E7:
In CE1 , a number of laminated sheets having a structure of "glass/EVA sheet/PVF film/PET film/PVF film" were prepared as follows. First, a layer of PET film was laminated betweentwo layers of PVF film, that were not heat
pressed, usingLiofol LA2692 polyurethane adhesives and hardener UR7395 (both purchased from Henkel AG&Co., Germany) at aratio of 1 1 :1 to form a laminated tri- layer film of "PVF film/PET film/PVF film". Then, one layer of EVA sheet(7X10 cm) was positioned between one layer of a 3.2 mm thick glass sheet (7x10 cm) and one layer of the laminated tri-layer film of "PVF film/PET film/PVF film" (7x12 cm) to form a multi-layer pre-lamination structure, which was subject to vacuum lamination at 145°C and 1 atm for 15 minutes using a ICOLAM 10/08 laminator. In addition, about half way along the length of the pre-laminationstructure, a piece of fluorinated ethylene propylene(FEP) release film was positioned between the EVA sheetlayer and the adjacent tri-layer film layer of "PVF film/PET film/PVF film" prior to vacuum lamination process. This way, after removing the FEP release film, the tri-layer film layer of "PVF film/PET film/PVF film" would have a loose end that is not bonded to the EVA sheet layer. Then, along the length of each of the laminate sheets, two test strips (2.54 cm wide and 12cm long) were cut out and the bonding strength between the EVA sheet and adjacent PVF film were measuredin accordance with ASTM D903-98 using an lnstron5566 tester (purchased from Instron (U.S.A.)). The bonding strength of a total of6strips as so prepared was measured and their average was calculated and tabulated in Table 1.
In each of CE2 and E1 -E7, a number of laminated sheets having a structure of "glass/EVA sheet/heat pressed PVF film/PET film/heat pressed PVF film" were prepared following substantially the same method as those disclosed above in CE1 , with the exception that heat pressed PVF films were used in place of the non-heat pressed PVF films of CE1. The various heat pressed PVF filmsused in each of CE2 and E1 -E7 were obtained by placing a layer of PVF film in an infra red (IR) oven (Model Number 10831010, purchased from Shanghai Yuejin Medical Instruments Factory, China) for a certain period of time, which was followed by passing the heated PVF films through a pair of heated nip rolls. The temperature and residence time of the IR oven and the temperature, residence time, and pressure of the heated nip rolls used in preparing these various heat pressed PVF films are listed in Table 1. Moreover, the bonding strength between the EVA sheets and the adjacent heat pressed PVF films in each of CE2 and E1 -E7 were determined by the same method used in CE1 and are tabulated in Table 1.
TABLE 1
Figure imgf000015_0001
Note: • In examples E2 and E5, the PVF film underwent nip roll pressing without the heat treatment by IR oven;
• 2 NA stands for "not applicable";
• 3 The bonding strength measured was between the EVA sheet and the adjacent PVF film (heat pressed PVF films in CE2 and E1 -E7).
As demonstrated hereabove, heat pressed PVF films had improved bonding strength with the EVA sheets (ranging from 29.8-76.2 N/cm in E1 -E7) compared to that of the untreated PVF films (10.1 N/cm in CE1 ).

Claims

What is Claimed is:
1. A method for obtaining a heat pressed fluoropolymer film or sheet that has improved bonding strength to a polyolefin film, the method comprising, (i) providing an oriented fluoropolymer film or sheet that consists essentially of fluoropolymer; and (ii) heat pressing the oriented fluoropolymer film or sheet by a heat press means, wherein the heat press means is set at such conditions that the oriented fluoropolymer film or sheet receives a pressure of 0.01 -50 Kgf/cm2 and a heat of150°C-260°C.
2. The method of Claim 1 , wherein in step (ii), the heat press means is set at such conditions that the oriented fluoropolymer film or sheet receives a pressure of 0.1 -50 Kgf/cm2, or preferably 0.5-50 Kgf/cm2, or more preferably 0.5-30 Kgf/cm2 and a heat of 150°C-245°C, or preferably 160°C-220°C, or more preferably 160°C-200°C.
3. The method of Claim 1 or 2, wherein the heat press means is a pair of hot flat plates and in step (ii), the oriented fluoropolymer film or sheet isheat pressed between the hot flat plates for 0.1 -30 sec, or preferably 0.5-30 sec, or more preferably 0.5-20 sec.
4. The method of any of Claims 1 -3, wherein the heat press means comprises at least one pair of heated nip rolls and in step (ii), the oriented fluoropolymer film or sheet ispassed through the at least one pair of heated nip rolls that areset at a line speed of 0.01 -100 m/min, or preferably 0.1 -50 m/min, or more preferably 0.5-30 m/min.
5. The method of any of Claims 1 -4, wherein, prior to step (ii), the oriented
fluoropolymer film or sheet is pre-heated to a temperature of 150°C-260°C, or preferably 150 °C-245°C, or more preferably 160°C-220°C, or yet more preferably 160°C-200°C.
6. The method of Claim 5, wherein, prior to step (ii), the oriented fluoropolymer film or sheet is pre-heated by infra-red heat, air heat, flame heat, electron beam, or laser; or preferably the oriented fluoropolymer film or sheet is heated by an infra-red oven.
7. The method of any of Claims 1 -4, wherein, the step (ii) of heat pressing is repeated 2-50 times, or preferably 2-10 times.
8. The method of any of Claims 1 -7, wherein the fluoropolymer is derived from fluoromonomersselected from the group consisting of vinyl fluorides, vinylidene fluorides, tetrafluoroethylenes, hexafluoropropylenes, fluorinated ethylene propylenes, perfluoroalkoxys,chlorotrifluoroethlyenes, and combinations of two or more thereof.
9. The method of Claim 8, wherein the fluoropolymer is selected from the group consisting of polyvinyl fluorides, polyvinylidene fluorides, and combinations thereof; or preferably the fluoropolymer is selected from polyvinyl fluorides.
10. A heat pressed fluoropolymer film or sheet prepared by the method of any of Claims 1 -9.
1 1 . The heat pressed fluoropolymer film or sheet of Claim 10, wherein the heat pressed fluoropolymer film or sheet is laminated to a polyolefin film or sheet, and the bonding strength between the heat pressed fluoropolymer film or sheet and the polyolefin film or sheet,measured according to ASTM D903-38, is at least 17 N/cm, or preferably at least 20 N/cm, or more preferably at least 40 N/cm.
12. The heat pressed fluoropolymer film or sheet of Claim 1 1 , wherein the
polyolefin film or sheet comprises a polyolefinic composition, and wherein the polyolefinic composition comprises a material selected from the group consisting of ethylene/vinyl acetate copolymers, ionomers, polyethylenes, ethylene/acrylate ester copolymers, acid copolymers, and combinations of two or more thereof; or preferably the polyolefinic composition comprises a material selected from the group consisting of ethylene/vinyl acetate copolymers.
13. A multi-layer fluoropolymer film or sheet comprising at least one layer of the heat pressed fluoropolymer film or sheet of any of Claims 9-12.
14. The multi-layer fluoropolymer film or sheet of Claim 13, which is in the form of a bi-layer film or sheet, and wherein the bi-layer film or sheet consists essentially of one layer of the heat pressed fluoropolymer film or sheet and one layer a polyester film or sheet that is laminated to the heat pressed fluoropolymer film or sheet.
15. The multi-layer fluoropolymer film or sheet of Claim 13, which is in the form of a tri-layer film or sheet, and wherein the tri-layer film or sheet consists essentially of two layers of the heat pressed fluoropolymer film or sheet and one layer of a polyester film or sheet that is laminated between the two heat pressed fluoropolymer film or sheet layers.
16. The multi-layer fluoropolymer film or sheet of Claim 14 or 15, wherein the polyester film or sheet consists essentially of polyester selected from the group consisting of polyethylene terephthalates, polybutylene terephthalates, polytrimethylene terephthalates, polyethylene naphthalates, and combinations of two or more thereof.
17. A solar cell module comprising one or a plurality of solar cells, a back
encapsulant sheet laminated to a back side of the solar cells, and a backsheet laminated to a back side of the back encapsulant sheet, wherein the back encapsulant sheet comprises a polyolefin, and wherein the backsheet comprises at least one layer of the heat pressed fluoropolymer film or sheet recited in any of Claims 10-12.
18. A solar cell module comprising one or a plurality of solar cells, a back
encapsulant sheet laminated to a back side of the solar cells, and a
backsheet laminated to a back side of the back encapsulant sheet, wherein the back encapsulant sheet comprises a polyolefin, and wherein the backsheet comprises a multi-layer fluoropolymer film or sheet recited in any of Claims 13-16.
19. The solar cell module of Claim 17 or 18, wherein the polyolefin comprising the back encapsulant sheet is selected from the group consisting of ethylene/vinyl acetate copolymers, ionomers, polyethylenes, ethylene/acrylate ester copolymers, acid copolymers, and combinations of two or more thereof, or preferably the polyolefin comprised in the back encapsulant sheet is selected from ethylene/vinyl acetate copolymers.
PCT/CN2012/076220 2011-06-07 2012-05-29 Solar cell backsheet with improved adhesion to encapsulant WO2012167705A1 (en)

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