Method for manufacturing photovoltaic panels by the use of a polymeric tri-layer comprising a composite getter system

Abstract

The present invention relates to a method for manufacturing thin-film photovoltaic panels by the use of a sealing means composed by a polymeric tri-layer comprising a composite getter system composed of a polymer with a low H.sub.2O transmission, having dispersed in its inside a H.sub.2O sorption material, and two outer polymeric layers with the composite getter system therebetween, as well as to a polymeric tri-layer for the manufacturing of photovoltaic panels.

Claims

1. A polymeric tri-layer for the manufacturing of photovoltaic panels, wherein the two outermost layers are composed of a polymeric material essentially without getter material, whereas the central layer is composed of a composite getter system for H.sub.2O sorption.

2. The polymeric tri-layer according to claim 1, in which said polymeric material that is essentially without getter material thereby provides an adhesion property quantifiable by a shear force resistance of at least 100 KPa.

3. The polymeric tri-layer according to claim 1, further comprising an additive material that is added to said outer polymeric layers for enhancing an adhesion property of said outer polymeric layers.

4. The polymeric tri-layer according to claim 1, wherein said layers have a softening or melting temperature lower than 170 C.

5. The polymeric tri-layer according to claim 1, wherein the central layer in contact with the outermost layers has a weight percentage of a H.sub.2O sorber inside said composite getter system being between 10 and 60 wt %.

6. The polymeric tri-layer according to claim 5, wherein the H.sub.2O sorber is selected from zeolites, silica gels, alumina and alkaline earth metal oxides.

7. The polymeric tri-layer according to claim 1, wherein said layers have a H.sub.2O transmission lower than 10 g m.sup.2d.sup.1 mm at 25 C. and 60% RH.

8. The polymeric tri-layer according to claim 1, in which said polymeric material that is essentially without getter material thereby provides a desired adhesion property.

9. The polymeric tri-layer according to claim 1, wherein the two outermost layers and central layer are composed of the same type of polymeric material and wherein the outermost layers have added silanes.

10. A polymer composition for manufacturing photovoltaic panels, capable of exhibiting a first stage and a second stage, wherein: in the first stage, the polymeric composition is a polymeric tri-layer, of which the two outermost layers are composed of a polymeric material essentially without getter material, whereas the central layer is composed of a composite getter system for H.sub.2O sorption; and in the second stage, the polymeric composition is an adhesive composite comprising H.sub.2O getter material distributed therein, the second stage being obtained by softening and reciprocal interdiffusion of the tri-layer upon heating the first stage to a temperature comprised between 100 C. and 170 C.

11. A polymeric tri-layer directed at separating an adhesion aspect and a H.sub.2O sorption aspect, the polymeric tri-layer comprising: a central layer composed of a getter material for H.sub.2O sorption; and a pair of outer layers that are substantially free of said getter material thereby providing a desired adhesion property.

12. The polymeric tri-layer according to claim 11, wherein the desired adhesion property is triggered or enhanced by heating.

13. The polymeric tri-layer according to claim 11, wherein the desired adhesion property is enhanced by addition of an additive.

14. The polymeric tri-layer according to claim 13, wherein addition of the additive enhances the desired adhesion property to obtain a lap shear resistance of at least 100 KPa.

15. The polymeric tri-layer according to claim 13, wherein the additive comprises at least one of a silane or a polyolefine functionalized by maleic anhydride grafting.

16. The polymeric tri-layer according to claim 11, wherein said getter material is limited to not higher than 1 wt % in said pair of outer layers.

17. The polymeric tri-layer according to claim 11, wherein said getter material in said central layer has a weight percentage between 10 and 60 wt %.

18. The polymeric tri-layer according to claim 11, wherein said getter material in said central layer has a weight percentage between 30 and 45 wt %.

19. The polymeric tri-layer according to claim 11, wherein said desired adhesion property accommodates attaching the polymeric tri-layer to one or more supports of a photovoltaic panel.

Description

(1) The invention will be illustrated, in the following with reference to the drawings, in which:

(2) the FIGS. 1A and 1B represent a cross-sectional view of a photovoltaic panel manufactured according to the present invention, respectively before (1A) and after (1B) the thermosealing process;

(3) FIG. 2 shows an enlarged detail of a polymeric tri-layer according to the present invention after being subjected to the thermosealing process; and

(4) the FIGS. 3A-3C represent EDX spectra that show the intensity of the calcium peak in various points of the tri-layer after the thermosealing process.

(5) In the drawings the dimensions and the dimensional relations between the various elements, particularly with reference to their thicknesses, are not correct, but have been distorted for the sake of better comprehensibility of the figures. Further some constitutive elements of a photovoltaic panel, as for example electrical feedthroughs and connections, have not been shown because they are not relevant for the description of the present invention.

(6) In the realization of the polymeric tri-layer object of the present invention is important that the two outermost polymeric layers are essentially free of H.sub.2O sorbing material, meaning that the amount of sorbing materials or moisture getters in such layers is not higher than 1 wt %. This means that the presence of small amounts of sorbing material may be tolerated since not impairing the adhesive properties of said outermost polymeric layers.

(7) FIG. 1A shows in cross section a photovoltaic panel 10, to which the method of the invention is applied, before the operation of thermosealing of the supports. In this drawing the photovoltaic element 11 has been shown in contact with the lower support 12. This element is encapsulated by a polymeric material 14 that fills the inner volume of the photovoltaic panel delimited by the lower support 12, by the upper support 13 and by the tri-layer formed of the composite getter system 15 and of the outer polymeric layers without getter material 16, 16; this tri-layer is arranged along the edge of the panel.

(8) The polymer used for the outer layers 16, 16 of the tri-layer must have good adhesive properties towards the materials that form the upper and lower support of the photovoltaic panel, i.e. have an adhesion to the supports of the panel quantifiable by a shear force resistance of at least 100 KPa as measured according to the ASTM C961-06 procedure.

(9) The information on the lap shear force (often referred also as lap shear strength) of different materials is readily available to a person skilled in the art and could be easily retrieved by a plurality of sources, see for example the publication Moisture transport, adhesion and corrosion protection of PV module packaging materials by Jorgensen et al, published in 2006 in the Solar energy Materials & solar Cells, pages 2739-2775, or the 2007 edition of the Physical Properties of Polymers Handbook edited by Springer New York; moreover the information on the lap shear force of a material to be employed in a production process of photovoltaic elements is often specified by the material supplier.

(10) Typical materials for the manufacturing of the transparent support are vitreous materials, whereas the other support of the panel can be made of various materials, among the most common ones of which there are vitreous materials; if this support should also have the characteristic of flexibility, an aluminized polymeric foil could be used. For example, for the polymeric foil, polyvinyl fluorides can be used, among which the use of Tedlar is preferred, both used alone or in combination with other elements as polymeric foils, or aluminized polyethylene terephthalate (PET).

(11) FIG. 1B shows the photovoltaic panel 10 after the process of thermosealing. In this case the constituent elements of the tri-layer, i.e. the outer polymeric layers 16, 16 and the composite getter system 15 arranged in the centre of the tri-layer, soften and melt into each other and give rise to a composite 17 inside of which the H.sub.2O sorber is distributed.

(12) This new composite getter system 17 that is formed following the softening and melting process shows a good adhesion to the supports of the photovoltaic panel in so far as the adhesive characteristics are determined essentially by those of the outer layers of the tri-layer, whereas the diffusion of the sorbing material ensures the formation of a barrier active against the ingress of H.sub.2O inside the panel.

(13) The thermosealing of the panel occurs by heating at a temperature between 100 C. and 170 C. The manufacturing process of a photovoltaic panel foresees also a process of thermosealing, in this regards two main types of processes are used, one foresees the use of a vacuum laminator, while the other foresee the use of an autoclave. In both cases the thermosealing is usually carried out between 100 and 170 C. This process results in the softening or melting of the encapsulating polymer.

(14) As shown in FIG. 1B, as a result of this operation it is possible that the thickness of the composite getter 17 obtained by the softening, melting and the reciprocal interdiffusion of the layers of the tri-layer results smaller than the starting thickness of the polymeric tri-layer as a consequence of the combined action of the weight of the upper support and the heating. Further it is possible that the composite getter 17 slightly stands out, both towards the inside and towards the outside of the panel 10, in which case the quantity of material that outflows from the panel is removed. This effect may be present with both the two different types of thermosealing processes.

(15) With regard to the properties required for the used polymers, these must have a softening or melting temperature lower than 170 C.; preferably this temperature is lower than or equal to 150 C.; they must further have a low MVTR for H.sub.2O, preferably lower than 10 g m.sup.2 d.sup.1 mm at 25 C. and 100% relative humidity. Polymers useful for carrying out the invention are for example EthylVinylAcetate (EVA), polyethylene of low (LDPE), medium (MDPE) and high (HDPE) density, polyether block amides (PEBA), ionomeric resins such as Surlyn commercialized, by DuPont, ethylene-acrylic, acid copolymers as for example Lucalen commercialized by Basell, polyvinylidene fluoride (PVDF), PolyVinylButyral (PVB) and polyvinylidene chloride (PVDC) such as Saran commercialized by DOW Chemicals, Ethylene-Propylene Rubbers (EPR), Ethylene Propylene Diene Monomer Rubbers (EPDM) and butyl rubbers.

(16) It is pointed out that the H.sub.2O transmission, like also the softening or melting temperature, are not linked univocally to the type of polymer but are functions of various parameters, among which the composition is one of the most relevant ones; thus it is important not to choose the polymer on the basis of the class it belongs to but on the basis of its H.sub.2O transmission and the softening or melting temperature, this information being easily available to an expert in the field. For example, as far as the transmission characteristics of EVA are concerned, this information is available in the article by Marais et al. Permeation and Sorption of Water and Gases through EVA Copolymers Films published in 2002 in Mat. Res. Innovation, Vol. 6, pp. 79-88.

(17) With regard to the characteristics of the polymers used for the outer layers of the tri-layer, these must further have good adhesive characteristics to the supports of the photovoltaic panel, quantifiable by a shear force resistance measured according to the ASTM C961-06 procedure of at least 100 KPa. In any case, independently by the specific characterizing method adopted for assessing the lap shear resistance, it has to be intended that the good adhesion properties of the outer polymeric layers are triggered or enhanced by thermal processes, such as the one specified in ASTM C961-06 or as the ones deriving from the thermosealing process of the photovoltaic module.

(18) The adhesive properties of these polymers can be improved by adding suitable additives such as for example silanes or polyolefines functionalized by maleic anhydride wafting.

(19) In a preferred embodiment the tri-layer is composed of the same type of polymer, with the only difference that the outer polymeric layers of the tri-layer have added silanes to improve the adhesive characteristics thereof in order to reach the lap shear resistance of at least 100 KPa. In an even more preferred embodiment this common polymeric material is the same as the polymeric material used for encapsulating the photovoltaic element; in this latter case the use of EVA result to be very advantageous.

(20) The above-mentioned supports of the photovoltaic panel can be made from glass or, in the case it is desired to obtain flexible panels, can be made from plastic materials such as ethylene tetrafluoroethylene (ETFE).

(21) In the case in which the polymeric materials that form the initial tri-layer are different from each other, it is important that the polymeric material of the composite getter system is, at its softening or melting temperature, miscible with the polymeric material forming the outer layers of the tri-layer.

(22) With regard to the materials for H.sub.2O sorption are concerned, zeolites, silica gels, alumina and alkaline earth metal oxides can be used.

(23) The composite getter system for the H.sub.2O sorption that forms the central part of the tri-layer has a weight percentage of the H.sub.2O sorbing material comprised between 10 and 60 wt %, preferably comprised between 30 and 45 wt %. It is important that the quantity of H.sub.2O sorbing material must be not less than 10% by weight, in order to have an efficient barrier. It is important to remember that this quantity by diffusing also in the outer polymeric layers of the tri-layer during the thermosealing process creates the barrier active against the entry of H.sub.2O into the photovoltaic cell, whereby the medium concentration of the sorbing material of the composite getter system 17 obtained after the softening and melting of the starting tri-layer is necessarily lower with respect to the starting composite getter system one, resulting in the lower limit given above.

(24) The inventors have determined that when operating under the conditions of the invention the sorbing material is diffusing also in proximity of the supports of the photovoltaic panel, thus creating a barrier active against the ingress of H.sub.2O inside the device, without however compromising the adhesive characteristics that the interface between the two supports must have.

(25) In a second aspect thereof the invention consists in a polymeric tri-layer for the manufacturing of photovoltaic panels, characterized in that the outermost are essentially without getter material, whereas the innermost layer is a composite getter system composed of a polymer with inside a dispersion of a moisture getter material.

(26) Moreover, the outermost layers have good adhesive characteristics, quantifiable by a shear force resistance of at least 100 kPa.

Example 1

(27) A tri-layer is manufactured using two outer layers with a thickness of 0.6 mm of EVA comprising an additive belonging to the family of silanes that acts as adhesion promoter. The inner layer with a thickness of 1.4 mm is composed of EVA loaded by 40% in weight with calcium oxide powder. The previous tri-layer is used as interface between two glass supports simulating thus the structure of a photovoltaic panel. The sample obtained in this way is subsequently subjected to a light compression and a thermal process at 150 C. for 30 minutes, similar to that of thermosealing in a photovoltaic panel. The shear force resistance for this sample after a thermosealing performed as described above is 5200 KPa.

(28) FIG. 2 shows the representation of a photograph of the contact region between the lower support 12 and the tri-layer after that this has been subjected to the thermal treatment of thermosealing that has caused the softening and melting of the layers, showing the parts without sorbing material 14, and the parts 17 that contain said sorbing material.

(29) It has been chosen to show the photographic representation and not to include the original photograph due to legibility problems connected to the printing quality of the latter one.

Example 2

(30) A sample prepared in a similar way to what described in example 1 is analysed by means of an energy dispersive X-ray micro-analyser (Oxford Energy Dispersive X-Ray Spectroscopy) connected to a scanning electron microscope (SEM Leica Stereoscan 420) in order to obtain a qualitative and quantitative evaluation of the chemical composition of the sample subjected to analysis. The spectra obtained in correspondence of the central section, of the upper support and of the lower support are shown respectively in the FIGS. 3A, 3B, 3C.

Example 3

(31) A tri-layer is manufactured in a manner similar to sample 1 but using as central layer MDPE loaded with 35% in weight with calcium oxide powder. The thickness of the central layer is 0.75 mm. Also this tri-layer is used as interface between two glasses supports simulating thus the structure of a photovoltaic panel. The sample obtained in this way is subjected to a light compression and a thermal process at 150 C. for 30 minutes, similar to that of thermosealing in a photovoltaic panel.

(32) The shear force resistance for this sample after a thermosealing performed as described above is 4300 KPa.

Example 4

Comparative

(33) A 2 mm mono-layer is manufactured in a manner similar to the central layer of example 3. The higher thickness of this example enables to make a direct comparison with example 3 tri-layers. This monolayer is used as interface between two glasses supports simulating thus the structure of a photovoltaic panel. The sample obtained in this way is subjected to a light compression and a thermal process at 150 C. for 30 minutes, similar to that of thermosealing in a photovoltaic panel.

(34) In this case the sample did now show any adhesive feature and the two glasses were easily separated.

(35) FIG. 2 shows that operating under the conditions of the invention the sorbing material diffuses also in correspondence of the vitreous support, i.e. it is possible, starting from an initial situation in which the concentration of the sorbing material is highly non-uniform (being present only in the central region of the tri-layer), to arrive at a situation where the sorbing material is diffused also in correspondence of the vitreous surfaces, thereby eliminating the preferential regions for the ingress of H.sub.2O in the system. The efficiency of this solution can actually be deduced from FIG. 2, where it can be seen how the particles of the H.sub.2O sorber (in the parts 17) arrive in contact with the vitreous support, thus eliminating preferential paths for the ingress of H.sub.2O.

(36) The FIGS. 3A, 3B, 3C show that there are significant quantities of sorber, represented by the calcium peak, also in correspondence to the supports, as confirmation of the fact that the method of the invention allows to obtain a composite getter with good barrier properties along the total of its thickness from the softening and melting of a starting polymeric tri-layer characterized by the presence of a composite getter layer arranged in its central part.