Photovoltaic module comprising insulation layer with silane groups

Abstract

The present invention relates to a photovoltaic module comprising a solar cell element and an insulation material laminated to at least one side of the solar cell element wherein the insulation material comprises an olefin copolymer which comprises silane group-containing monomer units, to a process for the production of such a photovoltaic module, and to the use of an olefin copolymer which comprises silane group-containing monomer units for the production of an insulation layer of a photovoltaic module.

Claims

1. A photovoltaic module comprising: a solar cell element; a front cover; a back cover; and an insulation material laminated to (i) one side of the solar cell element and (ii) either the front cover or the back cover; wherein the insulation material comprises an olefin copolymer comprising hydrolysable silane groups introduced by grafting; wherein the olefin copolymer does not comprise an olefinically unsaturated carboxylic acid; wherein the olefin copolymer further comprises polar comonomer units selected from the group consisting of C.sub.1- to C.sub.6-alkyl acrylates, C.sub.1- to C.sub.6-alkyl methacrylates, and vinyl acetate; and wherein the polar comonomer units are present in an amount of from 2 wt % to 60 wt % of the olefin copolymer.

2. The photovoltaic module of claim 1, wherein the hydrolysable silane groups are introduced by reaction of the olefin copolymer with an unsaturated silane compound.

3. The photovoltaic module of claim 2, wherein the unsaturated silane compound is represented by formula (V):
R.sup.1SiR.sup.2.sub.qY.sub.3-q  (V) wherein R.sup.1 is an ethylenically unsaturated hydrocarbyl, hydrocarbyloxy or (meth)acryloxy hydrocarbyl group, R.sup.2 is an aliphatic saturated hydrocarbyl group, Y which may be the same or different, is a hydrolysable organic group; and q is 0, 1 or 2.

4. The photovoltaic module of claim 2, wherein the unsaturated silane compound is represented by formula (VI):
CH.sub.2═CHSi(OA).sub.3  (VI) wherein A is a hydrocarbyl group having 1-8 carbon atoms.

5. The photovoltaic module of claim 2, wherein the unsaturated silane compound is vinyl trimethoxysilane, vinyl triacetoxysilane, vinyl bismethoxyethoxysilane, vinyl triethoxysilane, gamma-(meth)acryloxy-propyltrimethoxysilane, or gamma-(meth)acryloxy-propyltriethoxysilane.

6. The photovoltaic module of claim 1, wherein the polar comonomer units are selected from the group consisting of C.sub.1- to C.sub.6-alkyl acrylates.

7. The photovoltaic module of claim 1, wherein the polar comonomer units comprise butyl acrylate.

8. The photovoltaic module of claim 1, wherein the polar comonomer units are present in an amount of from 10 wt % to 45 wt % of the olefin copolymer.

9. The photovoltaic module of claim 1, wherein the polar comonomer units are present in an amount of from 20 wt % to 40 wt % of the olefin copolymer.

10. The photovoltaic module of claim 1, wherein the olefin copolymer is an ethylene copolymer.

11. The photovoltaic module of claim 1, wherein the insulation material has a tensile modulus of at most 100 MPa, measured according to ISO 527-2 at 1 mm/min.

12. The photovoltaic module of claim 1, wherein the insulation material forms an insulation layer having a thickness of 0.1 to 5 mm on the at least one side.

13. The photovoltaic module of claim 1, wherein the olefin copolymer in present in an amount of at least 30 wt % based on the weight of the insulation material.

14. The photovoltaic module of claim 1, wherein the olefin copolymer further comprises butyl acrylate in an amount of from 10 wt % to 45 wt % of the olefin copolymer; and wherein the olefin copolymer is an ethylene copolymer.

15. The photovoltaic module of claim 1, wherein the polar comonomer units are selected from the group consisting of butyl acrylate, ethyl acrylate, and methyl acrylate.

16. A process for forming the photovoltaic module of claim 1 comprising: laminating a sheet of the insulation material to at least one side of a solar cell element.

17. A photovoltaic module consisting essentially of in sequence: a front cover; a first insulation monolayer; solar cell elements interconnected by a conducting material; a second insulation monolayer; and a back cover; wherein at least one of the first insulation monolayer and the second insulation monolayer comprises an olefin copolymer comprising hydrolysable silane groups introduced by grafting; and wherein the olefin copolymer further comprises butyl acrylate.

18. The photovoltaic module of claim 17, wherein the front cover comprises polycarbonate.

19. The photovoltaic module of claim 17, wherein the olefin copolymer does not comprise an olefinically unsaturated carboxylic acid.

20. The photovoltaic module of claim 17, wherein the hydrolysable silane groups are introduced by reaction of the olefin copolymer with an unsaturated silane compound selected from the group consisting of vinyl trimethoxysilane, vinyl triacetoxysilane, vinyl bismethoxyethoxysilane, vinyl triethoxysilane, gamma-(meth)acryloxy-propyltrimethoxysilane, or gamma-(meth)acryloxy-propyltriethoxysilane.

Description

(1) The present invention will be further illustrated by way of an example, and by reference to the following figures:

(2) FIG. 1: Results of peeling tests showing influence of vinyl trimethoxy groups on the adhesion strengths towards aluminium.

(3) FIG. 2: Results of peeling tests after ageing in water showing increase of adhesion strength after treatment in water.

(4) A sheet useable as an insulation layer of a photovoltaic module was produced starting from a composition with the components as given in Table 1.

(5) Three ethylene terpolymers (Terpolymers 1, 2 and 3) consisting of vinyl trimethoxysilane silane (VTMS) and methylacrylate (MA) or butylacrylate (BA), respectively, were produced on commercial tubular reactors at 2800 to 3300 bar and 240 to 280° C. peak temperatures.

(6) The polymers are described in Table 1 and compared with the EVA polymer Elvax 150 (Comparative Example 1), which has been the dominating material for insulation following the recommendations from the “Low cost flat plate silicon array project” carried out at the Jet Propulsion Laboratory. The project started 1975 and the outcome of the project is reported in Paul B Willis, “Investigation of materials and processes for solar cell encapsulation”, JPL Contract 954527 S/L Project 6072:1, 1986 (hereinafter denoted as ref JPL).

(7) TABLE-US-00001 TABLE 1 Comparative Test Terpolymer Terpolymer Terpolymer Example method 1 2 3 (Elvax 150) Monomer Ethylene Ethylene Ethylene Ethylene Comonomer 1 Type MA MA BA VA Amount, wt-% 31 20 17 32 Amount, mol-% 12.7 7.5 4.2 13.2 Comonomer 2 Type VTMS VTMS VTMS None Amount, wt-% 1.0 1.2 1.9 0 MFR.sub.2,16, ISO 1133 3.0 10.1 5.3 35 g/10 min (2.16 kg/190° C.) Density ISO 2781 958 928 957 Melt temperature, ° C. ISO 3146 70 76 96 63 Vicat softening point at 10N, ° C. ISO 306 <40 <40 36 Hardness Shore A ISO 868 53 88 65-73 Hardness Shore D ISO 868 10 28 24 Tensile modulus, MPa ISO 527-2, 2.4 20 31 2.2 1 mm/min Tensile strength at break, MPa ISO 37, 8 13 7.5 50 mm/min Brittleness temperature, ° C. ISO 812 <−70 <−70 −100 Volume resistivity, IEC 93 1.4 × 10.sup.14 2.5 × 10.sup.16 1 × 10.sup.14 Ω-cm at 20° C. Dielectric constant at IEC 250 3.21 2.69 2.6 50 Hz, 20° C. Power factor at 50 Hz, IEC 250 0.002 0.0009 0.003

(8) In Table 2 a summary of the key properties of insulation materials for PV-modules are collected based on the finding in the JPL report (ref JPL).

(9) TABLE-US-00002 TABLE 2 Characteristic Requirement Source Glass transition temperature <−40° C. (1) Total hemispherical light >90 incident (1) transmission at 400-1100 nm Hydrolysis None at 80° C., 100% (1) relative humidity Resistance to thermal oxidation Stable up to 85° C. Mechanical creep None at 90° C. Tensile modulus <20.7 MPa UV absorption degradation None at wavelength > 350 nm Hazing or clouding None at 80° C., 100 relative humidity Odor, human hazards None VA content >25% in order to reach an acceptable optical transmission of 89 to 92% Gel content >70% results in resistance in thermal creep with margin at 110° C.

(10) Table 1 shows that the incorporation of VTMS into the polymer chain and the replacement of vinylacetate by butylacrylate do not result in any significant differences in tensile strength, tensile modulus, electrical and low temperature properties as well as in hardness and melting point. It is also shown that these properties these can be controlled by the molar amount of comonomer 2. As outlined in ref JPL, the optical properties and tensile modulus are controlled by the crystallinity of the polymer and in order to reach an acceptable optical transmission the VA content should be >25 wt-% (>9.8 mol-%).

(11) In order to control the cross-linking properties, a tape of a thickness of 0.5 mm was produced on a Brabender tape extruder with a length/diameter ratio of 20. The temperature setting was 120-150-170° C. Prior to extrusion the different terpolymers were mixed with 5% of a masterbatch consisting of 96.5 wt-% of an ethylene butylacrylate copolymer (MFR.sub.2=7 g/10 min, BA content=17 wt-%), 1.5 wt-% of dodecylbenzene sulphonic acid as cross-linking catalyst and 2 wt-% 4,4-thiobis(2-tert.butyl-5-methylphenol) as stabilizer. The tapes were stored for 24 hours and 50% relative humidity at 23° C. prior to determination of cross-linking properties. The gel content was determined by putting milled tapes in boiling decaline for 10 hours.

(12) TABLE-US-00003 TABLE 3 Test Terpolymer Terpolymer Terpolymer method 1 2 3 Gel-content, wt-% Decaline 78 77 82 Hot-set test IEC 811-2-1 200° C., 0.20 MPa Elongation 20 25 15 Permanent set 0 0 0

(13) It can be concluded that all tapes based on any of Terpolymers 1 to 3 give suitable cross-linking degrees as insulation for photovoltaic modules after storage at ambient conditions.

(14) In order to evaluate the cross-linked tapes resistance to hydrolysis, the cross-linked tapes of Terpolymers 1 and 3 were stored in water. After the exposure, tensile strength at break, elongation at break according to ISO 37 (tensile testing speed=500 mm/min) and weight changes were evaluated. The results are reported in Table 4.

(15) TABLE-US-00004 TABLE 4 Weight Tensile Temp., Time, Change, strength Elongation, Sample Solution ° C. Days wt-% MPa % Terpol.1 None — — — 3.0 210 Terpol.1 Water 23 30 0.1 3.0 184 Terpol.1 Water 50 30 0.1 3.6 234 Terpol.1 Water 100 7 0.7 2.9 166 Terpol.3 None — — — 4.6 42 Terpol.3 Water 23 30 0.1 5.5 71 Terpol.3 Water 50 30 0 4.4 41 Terpol.3 Water 100 7 0.4 4.9 55

(16) The results show that Terpolymers 1 and 3 are resistant to hydrolysis in conditions relevant for photovoltaic modules.

(17) In order to evaluate the thermal stability of different possible copolymers, the following samples were heat treated in a thermogravimetic analyzer at 333° C. in a nitrogen atmosphere: EBA-4,3: Ethylene butyl acrylate (BA) copolymer, BA content 17 wt-% (4.3 mol-%), MFR.sub.2=6 g/10 min, EEA-4,8: Ethylene ethylacrylate (EA) copolymer, EA content 15 wt-% (4.8 mol-%), MFR.sub.2=8 g/10 min, EHEMA-1,8: Ethylene hydroxyl ethyl methacrylate (HEMA) copolymer, HEMA content 8 wt-% (1.8 mol-%), MFR.sub.2=1.5 g/10 min, EMA-5,7: Ethylene methyl acrylate copolymer, MA content 15.6 wt-% (4.8 mol-%), MFR.sub.2=15 g/10 min, EMMA-4,9: Ethylene methyl methacrylate (MMA) copolymer, MMA content 14.1 wt-% (4.9 mol-%), MFR.sub.2=8 g/10 min, EVA-6,7: Ethylene vinyl acetate (VA) copolymer, VA content 28 wt-% (6.7 mol-%), MFR.sub.2=8 g/10 min.

(18) The results of the thermal stability tests are shown in Table 5.

(19) TABLE-US-00005 TABLE 5 Sample Time at 333° C., min Weight loss, wt-% EBA-4,3 120 4.5 EEA-4,8 120 3.8 EHEMA-1,8 120 4.4 EMMA-4,9 120 2.7 EMA-5,7 120 2.6 EVA-6,7 90 13.1

(20) It is evident that the most preferred groups for crystallinity control of terpolymers intended photovoltaic module are methyl acrylate or methyl methacrylate.

(21) In order to study the influence of the vinyl trimethoxy silane groups on the polymers adhesion to polar substrates, 200 μm thick films of Terpolymer 3 and an ethylene vinyl trimethoxy silane copolymer containing 2 wt-% VTMS, MFR.sub.2=0.9 g/10 min were pressed by putting each film in between 150 μm thick aluminium foils and pressing together at 250° C. for 10 seconds at a pressure of 1.3 MPa.

(22) After one week at ambient conditions, the peel force was tested in an Instron 1122 by a 180° T-peel test, with a crosshead speed of 200 mm/min. The width of the test strip were 25 mm. The same test procedure were also performed on ethylene butylacrylate copolymers and ethylene acrylic acid copolymers. The latter is known to give excellent adhesion strength towards polar substrates. The results of the tests are shown in FIG. 1.

(23) The silane group-containing laminates were furtheron treated in water heated to 85° C. for 37 hours and compared with a laminate in which the plastic film was corona treated low density polyethylene (LDPE). The results of the tests are shown in FIG. 2. The tests show that vinyl trimethoxy silane groups have a dramatic influence on the adhesion strength which even increases after treatment in water.