The present invention relates to an electro-conductive backsheet comprising a thermoplastic backsheet, an aluminium layer and a metal coating for back contact photovoltaic cell technology. The present invention also relates to a process for the manufacturing of the electro-conductive backsheet. The invention further relates to a photovoltaic module comprising the electro-conductive backsheet.

Photovoltaic modules serve for electrical power generation from sunlight and consist of a laminate which comprises a solar cell system as the core layer. To form a photovoltaic module, solar cells grouped in series through appropriate electrical conductors called "ribbons", are typically encapsulated by an encapsulating material such as for example polyethylene (PE) with a high content of vinyl acetate, commonly known as EVA. The encapsulating material enclosing the solar cells serves as protection against mechanical and weathering-related influences.

The core layer is present between a surface layer and a base layer or back-sheet, to complete the photovoltaic module. The surface layer, or main surface of the module, typically made of glass, covers the surface of the module exposed to the sun and allows the solar light to reach the cells. The base layer or back-sheet carries out a multiplicity of tasks. It guarantees protection of the encapsulating material and the solar cells from environmental influences, while simultaneously preventing the electrical connections from oxidizing. Typically, the base layer or back-sheet prevents moisture, oxygen and other factors depending on the atmospheric conditions from damaging the encapsulating material, the solar cells and the electrical connections. The back-sheet also provides for electrical insulation for the cells and the corresponding electrical circuits.

Photovoltaic modules comprising back-contact cells such as EWT (emitter-wrap-through) or MWT (metal-wrap-through) or IBC (interdigitated back- contact) solar cells may comprise a conductive patterned back-sheet for contacting the electrical contacts on the rear surface of the solar cells. Typically, such a back-sheet comprises at least a polymer layer and a patterned conductive metal layer made of copper, aluminium or a combination of both. Backsheets comprising a combination of both are known under the name Durashield® and comprise a highly conductive aluminium foil laminate, with a proprietary copper skin exterior coating which is applied via physical vapour deposition (PVD). PVD means Physical Vapor Deposition - also known as PVD Coating and refers to a variety of thin film deposition techniques where solid metal is vaporized in a high vacuum and high temperature (600°C) environment and deposited on electrically conductive materials as a pure metal.

Durashield® back contact sheets typically comprise PVDF-PET-AI-proprietary Copper coating. These back-contact sheets provide a good dimension stability and high weatherability. It is known that fluor-containing polymers typically display a very low water vapor transmission rate (WVTR) due to their very apolar nature. The WVTR of PVDF is 0.75 g.mm/m ^A 2.day or from PVF is 1.3 g.mm/m ^A 2.day, PVF-PET-PVF backsheet [2.132 g/m ^A 2.day]. One would guess that this is the reason behind the very good damp heat performance of fluor-containing backsheets. The presence of fluor- containing polymers is however a disadvantage because fluor containing polymers are known as environmental unfriendly and they may cause toxic (HF) gasses when caught in a fire.

The object of the present invention is to provide halogen free electro- conductive backsheets comprising PVD treated aluminium (Al). It has however been found that PVD treated Al on a backsheet comprising halogen free thermoplastic polymers, will drive up the resistances of the contacts between the Al layer and an electro-conductive adhesive (ECA) to unacceptable values upon ageing at

85°C/85%RH. Due to the increase in the resistance of the contacts between the PVD treated Al layer and ECA the power output of the photovoltaic module decreases.

The object of the present invention is to solve the above-mentioned problems and to provide a halogen-free electro-conductive multilayer backsheet comprising PVD treated Al, that when used in a photovoltaic (PV) module results in a reduced power output decay upon damp heat ageing. Output decay is typically assessed with a Damp Heat Test, followed by a solar cell IV characterization. The purpose of this test is to determine the ability of the PV module to withstand the effect of long-term penetration of humidity. PV modules are subjected to temperature 85°C and relative humidity 85 % for 500 hrs, 1000 hrs and 2000 hrs in a climatic chamber. Damp-heat test can prove the reduction of durability resistance of PV modules to long- term effect of humidity. PV modules with damaged PV cells, after the test, have significantly decreased power output of PV module. Solar Cell l-V Characteristic

Curves show the current and voltage (l-V) characteristics of a particular photovoltaic (PV) cell, module or array giving a detailed description of its solar energy conversion ability and efficiency. Knowing the electrical l-V characteristics (more importantly P _max ) of a solar cell, or panel is critical in determining the device’s output performance and solar efficiency. This object has been achieved in that an electro-conductive backsheet is provided comprising a physical vapor deposited metal coating on an aluminium layer and a halogen free thermoplastic multilayer backsheet comprising at least one polyester layer and at least one polyolefin layer whereby the halogen free thermoplastic backsheet has an OTR below or equal to 60 cm3/m2.atm per day at 38°C, 0% Relative Humidity (RH).

It has surprisingly been found that the combination of a physical vapor deposited aluminium layer and a halogen free thermoplastic multilayer backsheet comprising at least one polyester layer and at least one polyolefin layer having an oxygen transmission rate (OTR) below or equal to 60 cm3/m2.atm per day at 38°C and 0% relative humidity results in reduced power output decay upon damp heat ageing. Moreover, it has been found that the performance of the backsheets is not governed by their WVTR, the performance is governed by the OTR instead.

OTR (oxygen transmission rate) as used herein means the steady state rate at which oxygen gas permeates through a film at specified conditions of temperature and relative humidity (RH). OTR values are expressed in cc/m2/atm/24 hr in metric (or SI) units. Standard test conditions are 73°F (38°C) and 0% relative humidity (RH). Examples of oxygen permeation numbers of several polymeric materials can be found in Packaging technology and Science 2003; 16:149-158, or in Food contact polymeric Materials, review (ISSN:0889-3144 )-RAPRA technology Ltd., 1992. Oxygen permeation of thermoplastic polymeric layer(s) can be modeled using a permeation calculation by in-series connection of individual polymeric layers with individual resistances for mass transport. OTR of a back-sheet is governed by the Oxygen permeabilities P1 , P2... (in cm3.mm/m2.day) of the polymer materials in the different polymeric layers with resp. thicknesses 11 , 12,... according to OTR=1/

(I1/P1 +I2/P2+...)

The halogen free thermoplastic multilayer backsheet as used in the present invention mentioned means a back-sheet, preferably comprising at least 2 and up to 8 halogen free thermoplastic layers.

The halogen free thermoplastic multilayer backsheet as used in the present invention has an oxygen transmission rate (OTR) below or equal to 60 cm3/m2.atm per day at 38°C, 0% RH. Preferably it has an oxygen transmission rate (OTR) below or equal to 40 cm3/m2.atm per day at 38°C, 0% RH, more preferably an oxygen transmission rate (OTR) below or equal to 20 cm3/m2.atm per day at 38°C, 0% RH. A thermoplastic is a plastic material or a polymer that becomes pliable or moldable above a specific temperature and solidifies upon cooling.

The conductive back-sheet according to the present invention comprises a PVD treated aluminium layer, whereby a metal coating is applied. The aluminium layer preferably has a thickness in the range of 20-200 pm. The metal coating preferably has a thickness in the range of 50 nm- 5 pm.

The halogen free thermoplastic polymer is selected from the group consisting of polyolefines, polyamides or polyesters or a combination thereof.

Examples of polyamides are PA6, PA66, PA610, PA612, PA10, PA810, PA106, PA1010, PA1 1 , PA1011 , PA1012, PA1210, PA1212, PA814, PA1014,

PA618, PA512, PA613, PA813, PA914, PA1015, PA11 , PA12 or a semi-aromatic polyamide, called a polyphthalamide (PPA). (The naming of the polyamides

corresponds to the international standard, the first number(s) giving the number of carbon atoms of the starting diamine and the last number(s) the number of carbon atoms of the dicarboxylic acid. If only one number is mentioned, this means that the starting material was amino-carboxylic acid, or the lactam derived therefrom.

Reference is made to H. Domininghaus, Die Kunststoffe and ihre Eigenschaften [The polymers and their properties], pages 272 ff. , VDI-Verlag, 1976.) Suitable PPAs are for example, PA66/6T, PA6/6T, PA6T/MPMDT (MPMD stands for 2- methylpentamethylenediamine), PA9T, PA10T, PA1 1T, PA12T, PA14T and

copolycondensates of these latter types with an aliphatic diamine and an aliphatic dicarboxylic acid or with aminocarboxylic acid or a lactam.

Other examples of suitable polyamides include the polyamide of 1 ,10- decanedioic acid or 1 ,12-dodecanedioic acid and 4,4'-diaminodicyclohexylmethane (PA PACM10 and PA PACM12), copolymers and blends thereof.

Examples of amorphous polyamide are polyamides of terephthalic acid and/or isophthalic acid and the isomer mixture of 2,2,4- and 2,4,4

trimethylhexamethylenediamine, polyamides of isophthalic acid and 1 ,6- hexamethylenediamine, copolyamides of a mixture of terephthalic acid/isophthalic acid and 1 ,6-hexamethylenediamine, optionally in a mixture with 4,4'- diaminodicyclohexylmethane, copolyamides of terephthalic acid and/or isophthalic acid, 3,3'-dimethyl-4,4'-diaminodicyclohexylmethane and laurolactam or caprolactam, (co)polyamides of 1 ,12-dodecanedioic acid or sebacic acid, 3,3'-dimethyl-4,4'- diaminodicyclohexylmethane, and optionally lauro-lactam or caprolactam,

copolyamides of isophthalic acid, 4,4'-diaminodicyclohexylmethane and laurolactam or caprolactam, polyamides of 1 ,12-dodecanedioic acid and 4,4'- diaminodicyclohexylmethane, (co)polyamides of terephthalic acid and/or isophthalic acid and an alkyl-substituted bis(4-aminocyclohexyl)methane homologue, optionally in a mixture with hexamethylenediamine, copolyamide of bis(4-amino-3-methyl-5- ethylcyclohexyl)methane, optionally together with a further diamine, and isophthalic acid, optionally together with a further dicarboxylic acid, copolyamides of a mixture of m-xylylenediamine and a further diamine, e.g. hexamethylenediamine, and isophthalic acid, optionally together with a further dicarboxylic acid, for example terephthalic acid and/or 2,6-naphthalenedicarboxylic acid, copolyamides of a mixture of bis(4- aminocyclohexyl)methane and bis(4-amino-3-methylcyclohexyl)methane, and aliphatic dicarboxylic acids having 8 to 14 carbon atoms, and polyamides or copolyamides of a mixture comprising 1 ,14-tetradecanedioic acid and an aromatic, aryl-aliphatic or cycloaliphatic diamine.

Examples of polyolefins are polyethylene, EVA, EVOH or preferably a polypropylene. The polypropylene may in principle be of any customary commercial polypropylene type, examples being isotactic or syndiotactic homopolypropylene, a random copolymer of propylene with ethylene and/or but-1-ene, a propylene-ethylene block copolymer and the like. The polypropylene may be prepared by any known process, for example by the Ziegler-Natta method or by means of metallocene catalysis. It is possible to combine the polypropylene with an impact-modifying component, such as EPM rubber or EPDM rubber or SEBS.

More suitable polypropylenes are flexible polypropylenes (FPP). FPP is a mechanical or reactor blend of polypropylene (homo or copolymer) with EPR rubber (ethylene propylene rubber). Examples of such reactor blends are Hifax® CA 10 A, Hifax® CA 12, Hifax® CA7441A, supplied by LyondellBasell, or thermoplastic vulcanizates like Santoprene®. Santoprene is based on blend of polypropylene with EPDM rubber and is partly crosslinked. Examples of mechanical blends are blends of polypropylene with elastomers such as Versify® 2300.01 or 2400.01 (supplied by Dow) Another type of mechanical blend is polypropylene with LLDPE (linear low densitiy polyethylene) or VLDPE (very low-density polyethylene) plastomers (like Queo® 0201 or Queo® 8201 supplied by Borealis Plastomers), or copolymers of Ethylene with a polar co-monomer such as vinyl acetate or alkyl acrylates.

EVOH is a copolymer of ethylene and vinyl alcohol and is referred to as EVOH. A multiplicity of types is commercially available. EVA is a copolymer of ethylene and vinyl acetate and is referred to as EVA. Examples of thermoplastic polyesters include linear thermoplastic polyesters such as polyethylene terephthalate (PET), polypropylene terephthalate (PPT), polybutylene terephthalate (PBT), polyethylene 2,6-naphthalate (PEN), polypropylene 2,6-naphthalate (PPN) and polybutylene 2,6-naphthalate (PBN). It is possible to combine the polyesters with an impact-modifying component. The impact modifying component may comprise an elastomer such as for example chosen from the group consisting of EPDM, SBS, SEBS, ethylene-propylene elastomers such as EPDM, styrene-butadiene elastomers such as SBS or SEBS., such as EPM rubber or EPDM rubber or SEBS. The elastomer may comprise functional groups that bond chemically and/or interact physically with the polyester and wherein the elastomer constitutes a dispersed phase. The functional groups may be chosen from the group consisting of anhydrides, acids, epoxides, silanes, isocyanates, oxazolines, thiols and/or (meth)acrylates. Preferably the functional groups are epoxides. The halogen free thermoplastic polymeric layers may comprise additives known the art. Preferably the halogen free thermoplastic polymeric layers comprise at least one additive selected from UV stabilizers, UV absorbers, anti- oxidants, thermal stabilizers and/or hydrolysis stabilizers. When such additives stabilizers are present, a polymeric layer may comprise from 0.05-10 wt.% additives more preferably from 1- 5 wt.% additives, based on the total weight of the polymer.

White pigments such as talc, mica, Ti02, ZnO or ZnS may be added to the halogen free thermoplastic layers to increase backscattering of sunlight leading to increased efficiency of the PV module. Black pigments such as carbon black or iron oxide may be added for esthetic reasons but also for UV adsorption.

The present invention also relates to a process for the manufacturing of the electro-conductive back-sheet according to the present invention.

In one embodiment, the electro-conductive back-sheet can be manufactured comprising the steps of

(a)providing an aluminium layer and a metal coating applied via physical vapor deposition (PVD)

(b)providing a halogen free thermoplastic back sheet comprising at least one polyester layer and at least one polyolefin layer such that the back-sheet has an OTR below or equal to 60 cm3/m2.atm per day, at 38°C and 0% relative humidity (RH)

(c)co-extrusion / lamination or adhesion of the aluminium coated metal layer (a) and the halogen free thermoplastic back sheet (b). Preferably the metal coating is applied on top of the aluminium layer. The polymeric layers of the halogen free thermoplastic multilayer back sheet (b) can be laminated, extruded- laminated or co-extruded. Preferably the polymeric layers are co-extruded.

The metal coating can also be applied to both sides of the aluminium layer. The Advantage of applying the metal coating at both sides is that it will make the attachment of a junction box less complex. Typically, a junction box is attached to the rear of a back-contact module made with a conductive back-sheet by locally removing the polymer layers of the back-sheet mechanically, thermally or with a laser and then soldering wires or cables to the exposed metal. With a metal layer in the conductive back-sheet, the wires can be soldered directly to the metal. In the case of aluminium as the conductor, soldering directly is not possible due to the presence of oxides on the surface of the aluminium. If, for example PVD copper is applied to both sides of the aluminium sheet, direct soldering of the wires for the junction box is possible. In the case that copper is only applied to the cell side of the aluminium sheet, an additional treatment or process is needed to allow soldering of the wires for the junction box.

In a second embodiment, the electro-conductive back sheet can be manufactured via the following steps:

(a) providing an aluminium layer on a halogen free thermoplastic back sheet

comprising at least one polyester layer and at least one polyolefin layer having an OTR below or equal to 60 cm3/m2.atm per day, at 38°C and 0% relative humidity (RH).

(b) providing a metal coating on the aluminium layer containing polymeric back sheet of (a) via physical vapor deposition

In the second embodiment the aluminium layer in step a) is adhered to the halogen free thermoplastic back sheet by an adhesive. Examples of adhesives are single layer adhesives or dual layer adhesive systems. Examples of single layer adhesives are polyurethane, acrylate-based polymers or polyolefins. An example of a dual adhesive system is based on an adhesion layer comprising a modified polyolefin and straight- chain low-density polyethylene (LLDPE) whereby the percentage of the mass of the modified polyolefin to the mass of the LLDPE is 43 percent or more and an adhesive layer comprising polyethylene. The modified polyolefin is for example ADMER

(registered trademark and manufactured by Mitsui).

The above described embodiments may further comprise a patterning step. 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, mechanical milling or die cutting are preferably used.

The present invention further relates to a photovoltaic module comprising the electro-conductive back-sheet 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: (1 ) a transparent pane (representing the front sheet), (2) a front encapsulant layer, (3) a solar cell layer, (4) a back encapsulant layer, and (5) the electro-conductive back-sheet according to the present invention, representing the rear protective layer of the module.

The front sheet is typically a glass plate.

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.

The solar cell layer typically comprises back contact cells such as Emitter Wrap Through cells (EWP) or Metallization Wrap Through cells (MWT).

The photovoltaic module comprising the electro-conductive back- sheet according to the present invention surprisingly provides more stability, durability and a reduced power output decay upon damp heat ageing.

The present invention further relates to a method for manufacturing a photovoltaic module with a stack comprising at least a solar cell, an electro-conductive back-sheet, and a junction box, comprising:

-providing the solar cell being arranged as a back contacted solar cell with a front surface for receiving radiation and a rear surface provided with electrical contacts; -providing the electro-conductive back-sheet according to the present invention;

-patterning the conductive surface

-placing the solar cell with the rear surface facing the patterned conductive surface; -conductively contacting either each electrical contact of the solar cell with the corresponding contacting areas by an electro-conductive adhesive.

The present invention will now be described in detail with reference to the following non-limiting examples which are by way of illustration.

EXAMPLES

MEASUREMENTS

-Oxygen transmission rate (OTR [cc/(m2.atm.day)]) of polymer backsheets was determined according to ASTM D 3985 using an Mocon Ox-tran 2/21 at a temperature of 38°C and a relative humidity (RH) of 0%.

-Current (I) and Voltage (V) measurements (l-V) of the solar cells prior to module manufacturing were measured using SUNSIM flash-tester under standard testing conditions [1000W/m2, AM1.5 spectrum]

-The l-V characteristics of the modules were measured using a SUNSIM flash-tester. -Average power output difference of 3 mini-modules were determined as a function of damp-heat 85°C/85% RH ageing time. To pass the IEC 61215 norm the power decay must be less than 5% after 1000 hours damp-heat exposure. BACKSHEET MATERIALS

1. PA12-PP-PE backsheet: PA12 (35 micron) +PP (290 micron) +PE (35 micron) + 150nm single-side PVD Cu-coated Al (67 micron)

2. PA12-PP backsheet: PA12 (35 micron) + PP (325 micron) + 150nm single-side PVD Cu-coated Al (67 micron)

3. PET-EVA-PP-EVA backsheet: stabilized PET (55 micron) + EVA (30 micron) + PP

(150 micron) + EVA (30 micron) + 150nm single-side PVD Cu-coated Al (67 micron)

Comparative experiment l-ll

250 nm Cu-coated Al (67 micron) was laminated to backsheet material 1 and 2, using 6-micron Dow Adcote® 76P1-81 R_CR 865 polyurethane adhesive at room

temperature. The resulting electro-conductive backsheets (ECB) were allowed to cure at room temperature for 3 weeks. Subsequently the ECBs were patterned by mechanical milling suitable for 4 cell mini-modules (2x2 cells).

4 mini-modules were laminated using EMS conductive adhesive DB1588-4®, STR Photocap® 15580P/UF 200 pm punched encapsulant, JA Solar® cells measured with SUNSIM, STR Photocap® 15580P/UF 200 pm and 4mm glass (Scheuten Super white, not coated).

One mini-module was kept as a reference and 3 were submitted to damp-heat ageing (85° C/85% RH) for 500, 1000 (1xlEC 61215 norm), 1500, 2000 (2xlEC norm).

Current (I) and Voltage (V) measurements (l-V) were performed to determine the power output difference with the reference sample (no damp-heat treatment, stored in the dark).

Example 1

250 nm Cu-coated Al (67 micron) was laminated to backsheet material 3 using 6- micron Dow Adcote® 76P1-81 R_CR 865 polyurethane adhesive at room temperature. The resulting electro-conductive backsheet (ECB) was allowed to cure at room temperature for 3 weeks. Subsequently the ECBs was patterned by mechanical milling suitable for 4 cell mini-modules (2x2 cells).

2 mini-modules were laminated using EMS conductive adhesive DB1588-4®, STR

Photocap® 15580P/UF 200 pm punched encapsulant, JA Solar® cells measured with SUNSIM, STR Photocap® 15580P/UF 200 pm and 4mm glass (Scheuten Super white, not coated).

One mini-module was kept as a reference and 1 was submitted to damp-heat ageing (85° C/85% RH) for 500, 1000 (1xlEC 61215 norm), 1500, 2000 (2xlEC norm).

Current (I) and Voltage (V) measurements (l-V) were performed to determine the power output difference with the reference samples (no damp-heat treatment, stored in the dark). Results:

In table 1 results are given on Power output difference (POD [%]) of 2 mini-modules (2x2 cells) against reference sample at different damp-heat ageing times (500h, 1000h, 1500h, 2000h) Table 1

The results show that PVD copper-coated aluminium, halogen-free electro-conductive backsheet (Example 1 ) requires an OTR below 60 cc/(m2.atm.day) to pass the 1xlEC 61215 norm (power decay below 5% after 1000 hours damp-heat exposure).