WO2021191757A1

WO2021191757A1 - Improved lamination process

Info

Publication number: WO2021191757A1
Application number: PCT/IB2021/052318
Authority: WO
Inventors: Jiaying Ma; Herve E. Deve; Thomas K. ELLINGHAM, Jr.; Jay M. Jennen; Scott R. Meyer; Christopher M. SCHENIAN
Original assignee: 3M Innovative Properties Co
Current assignee: 3M Innovative Properties Co
Priority date: 2020-03-27
Filing date: 2021-03-19
Publication date: 2021-09-30
Anticipated expiration: 2022-09-27

Abstract

A method of applying an adhesive to a substrate to mitigate entrapment of air bubbles during a vacuum lamination process is described. The method comprises contacting the adhesive to a surface of the substrate and applying a periodic force to the adhesive along the longitudinal direction creating first regions having a first adhesion strength to the substrate and second regions having a second adhesion strength to the substrate, wherein the second adhesion strength is less than 90% of the first adhesion strength.

Description

IMPROVED LAMINATION PROCESS

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention is directed to an improved lamination process. In particular, a method of applying an adhesive to a substrate to mitigate entrapment of air bubbles during a vacuum lamination of a multilayer stack is described.

Background

Demand for renewable energy has grown substantially with advances in the technology and increases in global population. One promising energy resource is sunlight. Harnessing sunlight may be accomplished by the use of photovoltaic (PV) cells (also referred to as solar cells), which are used for photovoltaic conversion of sunlight to electrical current. PV cells are relatively small in size and are typically combined into a physically integrated PV module (or solar module) having a correspondingly greater power output than the individual PV cells of the module. PV modules are generally formed from a multilayer stack comprising two or more “strings” of PV cells surrounded by an encapsulant and enclosed by front and back panels, wherein at least one panel is transparent to sunlight. The multilayer stack is laminated together to form the PV module which provides mechanical support for the individual PV cells and protects them against damage due to environmental factors such as wind, snow, and ice. The PV module is typically fitted into a metal frame, with a sealant covering the edges of the module engaged by the metal frame. The metal frame protects the edges of the module, provides additional mechanical strength, and facilitates combining it with other modules so as to form a larger array or solar panel that can be mounted to a suitable support.

However, when a plurality of PV cells are arrayed in the PV module, the total active surface area of the array (i.e., the front faces of the PV cells) is less than the total area of the PV module that is exposed to the sunlight, because the PV cells are arranged in the PV module with space between the adjacent PV cells and between the PV cells and the edge of the module. This arrangement reduces the efficiency of the solar cell since some of the sunlight impinging on the PV modules falls on the inactive areas that lie between the PV cells or border the entire array of cells in the module. The efficiency of the solar cell can be increased by providing a reflective light management material in the inactive areas of the PV module to reflect the sunlight impinging on these areas onto the face of the PV cells. These reflective light management materials can include an adhesive layer to facilitate positioning of the reflective light management materials prior to lamination.

Difficulties can arise during lamination of large articles such as PV modules, especially if some of the layers in the multilayer stack have complex or intermittent structures. The intermittent structures may act as dams that can prevent removal of entrapped air from the multilayer stack. Additionally, air can become entrapped within the layers of the PV module due to relative viscosity differences between the flowable materials, such as the encapsulants, adhesives, etc. used to create the PV module. The entrapped air can decrease interlayer and/or intralayer adhesion and create aesthetic issues.

Thus, there is a need for techniques which can facilitate removal of entrapped air during PV module fabrication.

SUMMARY

An improved lamination process is provided herein. In particular, in a first embodiment, a method of applying an adhesive to a substrate to mitigate entrapment of air bubbles during a vacuum lamination process is described. The method comprises contacting the adhesive to a surface of the substrate and applying a periodic force to the adhesive along the longitudinal direction creating first regions having a first adhesion strength to the substrate and second regions having a second adhesion strength to the substrate, wherein the second adhesion strength is less than 90% of the first adhesion strength.

In another embodiment, a laminated article can be formed by first creating a multilayer stack of the constituent layers, wherein the layers include a first substrate, a discontinuous structure comprising a member having an adhesive layer disposed on the member, and a flowable material layer. The discontinuous member can be attached to a surface of the substrate by first contacting the adhesive of the discontinuous member to a surface of the substrate and applying pressure to periodically tack down the adhesive layer of the discontinuous member onto the surface of the substrate. A flowable film layer can be laid on the substrate over the discontinuous member. The multilayer stack can be placed in a vacuum laminator and laminated to create a laminated article.

In a third embodiment, an exemplary method of forming a PV modules is described. The method comprises creating a multilayer stack that includes a backsheet; a light management material, wherein the light management material comprises flexible light redirecting film having a plurality of microstructures extending from a first surface of the film and a continuous adhesive layer disposed on a second surface of the film; an array of PV cells; at least one layer of a flowable encapsulant and a front sheet. The light management material is applied to the backsheet using pressure to periodically tack down the adhesive layer of the light management material onto the surface of the backsheet. A first encapsulant film is placed on the backsheet over the light management material and an array of interconnected PV cells is placed on top of the first encapsulant sheet. A second film encapsulant sheet is placed over the array of interconnected PV cells and a front sheet is placed on top of the second encapsulant sheet. The multilayer stack is heated to a temperature above the melt temperature of the first and second encapsulant sheets and a vacuum is applied to remove entrapped air from the multilayer stack. Finally, a laminating pressure is applied to press the layers of the multilayer stack together completing the PV module lamination process.

The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description that follows more particularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be further described with reference to the accompanying drawings, wherein:

Fig. 1 A shows a schematic cross-section of an exemplary PV module after lamination.

Fig. IB is an exploded view of the multilayer stack used to form the PV module of Fig. 1A.

Figs. 2A-2C are schematic diagrams illustrating the mechanism of air bubble entrapment in an adhesive layer during vacuum lamination of a multilayer stack.

Fig. 3 shows air bubbles that became trapped in an adhesive layer during vacuum lamination of a multilayer stack.

Fig. 4 illustrate an exemplary new variable pressure bonding method in accordance with the present invention.

Fig. 5 A and 5B show a portion of the pseudo PV module of example Clp before and after lamination.

Fig. 6 A and 6B show a portion of the pseudo PV module of example Exlp before and after lamination.

While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims. DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “forward,” etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments of the present invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized, and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.

Fig. 1A shows a schematic cross-section of an exemplary PV module 100 after lamination and Fig. IB is an exploded view of the multilayer stack used to form the PV module of Fig. 1 A. PV module 100 includes an array of spaced apart PV cells 110 arranged along a length direction and a width direction, an encapsulant material 120 surrounding the PV cells, and top and bottom substrates 130, 140. Tabbing ribbons (not shown) make electrical connections between the PV cells and are generally aligned along the length direction. Areas, such as areas around the perimeter of the module and between the PV cells 110 are photovoltaically inactive.

Any PV cell format can be employed in the PV modules of the present disclosure (e.g., thin film photovoltaic cells, CuInSe2 cells, a-Si cells, e-Si sells, and organic photovoltaic devices, among others). A metallization pattern is applied to the PV cells 110, most commonly by screen-printing of silver inks. This pattern consists of an array of fine parallel gridlines, also known as fingers (not shown). Electrical connectors or tabbing ribbons (not shown) are disposed over and typically soldered to the PV cells to collect current from the fingers.

The top or front side substrate 130 serves as a protective cover and is typically made of clear glass or a suitable plastic material that is transparent to solar radiation and the bottom or rear side substrate 140 serves as a support for the PV cells and can be made of the same or a different material as the top substrate.

Exemplary encapsulant materials 120 is disposed between the first and second substrates 130, 140 filling any gaps and encasing PV cells 110. The encapsulant material comprises a suitable light-transparent, electrically non-conducting material having an optical transmissivity of at least 50% or at least 80% averaged over the solar spectrum, e.g. from 380 to 1100 nm.

Some exemplary encapsulants include curable thermosets, thermosettable fluoropolymers, acrylics, ethylene vinyl acetate (EVA), polyvinyl butyral (PVB), polyolefins, thermoplastic urethanes, clear polyvinylchloride, and ionomers. One exemplary commercially available polyolefin encapsulant is available under the trade designation P08500™ from 3M Company (St. Paul, Minn.). Both thermoplastic and thermoset polyolefin encapsulants can be used. Exemplary polyolefin encapsulant materials are described in United States Patent Nos. 9,276,151 and 9,379,263, incorporated herein by reference in its entirety.

Encapsulant material 120 can be provided as discrete sheets that are positioned below and/or on top of the array of PV cells 110, with those components in turn being sandwiched between the first and second substrates 130, 140. Subsequently, the laminate construction is heated under vacuum, causing the encapsulant sheets to become liquefied enough to flow around and encapsulate the PV cells, while simultaneously filling voids in the space between the first and second substrates 130, 140. Upon cooling, the liquefied encapsulant solidifies. In some embodiments, the encapsulant material may additionally be cured in situ to form a transparent solid matrix. The encapsulant material adheres to first and second substrates 130, 140 to form a laminated PV module 100.

Strips of light management material 160 can be disposed in inactive areas 150 to redirect light toward the photovoltaically active PV cells 110. Light management material 160 can comprise a light redirecting film (LRF) such as are available from 3M Company (St. Paul, MN). In the exemplary embodiment shown in Figs. 1 A and IB, LRF includes a first layer 162 comprising a plurality of microstructures 163 that extend away from a plane of the film. A second layer 165 is disposed on and conforms to surface 163a of the microstructures of the first layer. Second layer 165 is configured to redirect sunlight impinging on the first layer. A third layer comprising an adhesive 170 is disposed on the film layer opposite the microstructures. The LRF may include an optional protective layer 169 disposed over the second layer.

In some embodiments, first layer 162 can comprise a base layer 164 and a microstructure layer 163. The base layer can be made from polymeric material such as cellulose acetate butyrate; cellulose acetate propionate; cellulose triacetate; poly(meth)acrylates such as polymethyl methacrylate; polyesters such as polyethylene terephthalate and polyethylene naphthalate; copolymers or blends based on naphthalene dicarboxylic acids; polyether sulfones; polyurethanes; polycarbonates; polyvinyl chloride; syndiotactic polystyrene; cyclic olefin copolymers; silicone-based materials; and polyolefins including polyethylene and polypropylene; and blends thereof. Particularly suitable polymeric materials for the first layer 164 are polyolefins and polyesters. In an alternative aspect, the film layer 162 can be formed of a single material. The microstructures/microstmcture layer 163 can have a generally triangular cross sectional shape. For example, the microstructures can have a substantially triangular prism shape, which refers to a prism shape having a cross-sectional area that is 90% to 110% of the area of largest inscribed triangle in the corresponding cross-sectional area of the prism. In some embodiments, the substantially triangular prism shape may have slightly rounded facets or a rounded peak. The triangular prisms may be symmetrical (having substantially equal facet lengths and facet angles) or may be asymmetrical (having unequal facet lengths and facet angles). The arrangement of the microstructures can be continuous or discontinuous and can include a repeating pattern, a non-repeating pattern, a random pattern, etc.

The second layer of the LRF is made of a reflective material appropriate for reflecting at least some of the sunlight that impinges on the reflective surface of the toward the air-module interface at an angle such that the reflected light undergoes total internal reflection and is reflected again towards the surface of PV cells 110 for absorption. Exemplary reflective materials can comprise metallic, inorganic materials or organic materials. In some embodiments, the second layer comprises a mirror coating.

Exemplary light directing films are described in United States Patent No. 10,205,041, United States Patent Publication No. 2019-0237603, and Patent Cooperation Treaty Application Nos. PCT/IB2019/060127 and PCT/IB2020/061704, each of which is incorporated herein by reference in their entirety.

The adhesive layer (i.e. the third layer 170) may be a thermoset or thermoplastic adhesive that is substantially transmissive to the sunlight, e.g., the adhesive layer can have a transmissivity of at least 50% or at least 80% for wavelengths between 380 nm and 1100 nm. According to some embodiments, the third layer 170 may have a melt flow index of between about 0.1 and 8 g/10 minutes, between about 0.1 and 10 g/10 minutes, between about 0.1 g/10 minutes and 20 g/20 minutes or between 0.1 and 30 g/10 minutes as measured using ASTM D1238 performed at 190°C with a 2.16 kg weight. The adhesive may be of an analogous chemistry to the encapsulant or it may be a different material.

In an exemplary aspect, the adhesive layer comprises a thermally activatable adhesive, such as a hot melt adhesive and/or a thermally activated cross-linkable adhesive. In some embodiments, the adhesive layer may comprise one or more of polyethylene (PE), polypropylene (PP), polyolefin (PO), ethylene vinyl acetate (EVA), polyvinyl butyral (PVB), a polymer of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride (THV), ethylene tetrafluoroethylene (ETFE), polyvinylidene fluoride (PVDF), polyurethane (PU), poly(methyl methacrylate) (PMMA), polyimide (PI), among other materials. In other embodiments, the adhesive material employed in the third layer 170 may be a polymer that cures through heat, a chemical reaction (e.g., two-part epoxy), and/or irradiation by electron beam or UV radiation, for example. When cured, the third layer material is transformed to a plastic or rubber by crosslinking, forming bonds between individual chains of the polymer. Polyethylene resin, ethyl vinyl acetate (EVA), polyurethane, acrylate, and two part silicones are examples of suitable adhesive materials for third layer 170.

For example, the adhesive material employed in the third layer 170 may be a polymer that cures through heat, a chemical reaction (e.g., two part epoxy), and/or irradiation by electron beam or UV radiation, for example. In a preferable aspect, the adhesive material in third layer 170 is thermally cured. When cured, the third layer material is transformed to a plastic or rubber by crosslinking, forming bonds between individual chains of the polymer. Polyethylene resin, ethyl vinyl acetate (EVA), polyurethane, acrylate, and two part silicones are examples of suitable materials for third layer 170.

In an exemplary aspect, the adhesive material can be a crosslinkable EVA adhesive comprising a thermal crosslinking agent such as an organic peroxide, a C-radical donor or azo compounds to facilitate thermal crosslinking of the EVA adhesive. Some exemplary peroxides include, for example, diacyl peroxides (such as, for example, dilauryl peroxide and didecanoyl peroxide), alkyl peresters (such as, for example, tert-butyl peroxy-2-ethylhexanoate), perketals (such as, for example, l,l-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane or l,l-di(tert- butylperoxy)cyclohexane), dialkyl peroxides (such as, for example, tert-butyl cumyl peroxide, di (tert-butyl) peroxide and di cumyl peroxide), C-radical donors (such as, for example, 3,4- dimethyl-3,4-diphenylhexane and 2,3-dimethyl-2,3-diphenylbutane), and azo compounds (such as, for example, 2,2'-azodi(2-acetoxypropane)). The thermal crosslinking agent is selected such that the EVA adhesive will cross-link in a short time which is compatible with conventional PV module assembly processes. The organic peroxide is typically provided at 0-10 parts by weight, preferably, 1-5 parts by weight per 100 parts EVA.

The adhesive material may optionally include a cross-linking agent to increase the degree of crosslinking in the adhesive. The cross-linking agent can be an allyl group-containing compound, a compound containing acryloxy group, methacryloxy group-containing compound. Exemplary allyl group-containing compounds include, for example, allyl isocyanurates, allyl phthalates, allyl fumarates, allyl maleates and the like. In addition, a compound containing acryloxy group, methacryloxy group-containing compound, acrylic acid derivatives or methacrylic acid derivative, for example, the ester can be used. Further, ethylene glycol, triethylene glycol, polyethylene glycol esters of poly functional alcohols and the like can be used as well. These cross-linking auxiliary agents can be used in up to 10 parts by weight per 100 parts EVA.

The adhesive material may also include one or more adhesion promoters. For example, organosilanes such as chloropropyl silanes, vinyltrichlorosilanes, vinyltriethoxysilanes, vinyl tris(methoxyethoxy)silanes, methacryloxypropyltrimethoxysilanes, (3, 4-ethoxy cyclohexyl) ethyltrimethoxysilanes, glycidoxypropyltrimethoxysilanes, vinyltriacetoxysilanes, mercaptopropyltrimethoxysilanes, aminopropyltriethoxysilanes and aminopropyltrimethoxysilanes can be used in up to 5 parts by weight or less per 100 parts EVA.

In addition, ultraviolet absorbers, hindered amine light stabilizer (HALS) and antioxidants may also be used in the adhesive material. UV absorbers can be selected to have a “UV cutoff’ of 310, 350, and 380 nm, respectively. HALS are light stabilizers rather than absorbers that scavenge radicals by production of nitroxyl radicals, including, for example, cyclic amines, secondary, tertiary, acetylated, N-hydrocarbyloxy substituted, hydroxy substituted, N-hydrocarbyloxy substituted, or other substituted cyclic amines which are further characterized by a degree of steric hindrance. Antioxidants can be selected from phenolic compounds, sulfur-based compounds, phosphorus-based compounds, amine-based compounds, hydrazine or the like.

In an alternative aspect, the adhesive material can be a crosslinked or partially crosslinked EVA hotmelt adhesive such as is described in United States Patent Publication No. 2018-0013027, incorporated herein by reference in its entirety. Crosslinking can be achieved by any method known in the art, including by the use of actinic radiation (e.g., UV and ebeam). In the case of photo-chemically induced crosslinking, the process can be aided by the use of photo initiators and other known catalysts. In other embodiments, the crosslinking occurs by thermal curing, or by a combination of any of the different cross-linking methods disclosed here and know in the art.

As mentioned previously, PV modules are formed by laminating the multilayer stack described above in order to bond the layers together. During conventional assembly of the multilayer stack, the light management material can be applied to the second substrate (i.e. the back sheet) with a curable adhesive material using a smooth, round roller to apply constant pressure to the light management materials and under hot air to promote tackiness in the adhesive. In an exemplary embodiment, the curable adhesive material is a thermally curable EVA based adhesive. Optionally, heat can also be applied to the second substrate to promote adhesion of the light management material. Once the light management material has been adhered to the second substrate, the remaining components (encapsulant layers, PV cells and the first substrate) are added on top of the second substrate to form the multilayer stack. The multilayer stack is then placed in a heated vacuum chamber at a temperature above the melting point of the encapsulant and the adhesive material to remove air between the layers. The temperature of the vacuum chamber is typically about 128°C, while the melt temperatures of the encapsulant and adhesive materials are usually about 80°C. The entrapped air is pulled out of the multilayer stack by the vacuum. Next, pressure is applied via a bladder with heat to cure/crosslink the adhesive materials.

While a goal of this process is to remove all of the entrapped air from the multilayer stack, air bubbles 180 can become trapped in the adhesive due to the viscosity mismatch between the encapsulant and adhesive materials. Figs. 2A-2C schematically illustrate how the bubbles become entrapped in the adhesive layer during vacuum lamination of the multilayer stack. Specifically, Fig. 2A illustrates bubble formation that occurs due to air that is entrapped in the encapsulant material when the encapsulant material is heated above its melting temperature. Vacuum is then applied to the multilayer stack to pull the entrapped air out of the stack, the air bubbles 180 will move towards the edges of the multilayer stack (see Fig. 2B) as indicated by directional arrows 199, 198. While air bubbles in the bulk phase of the encapsulant can escape, air bubbles 180, which move into the adhesive layer 170 attaching the light management material to the substrate as they migrate to the edge of the multilayer stack, can become stuck in the adhesive layer 170 as the adhesive material cures as shown in Fig. 2C.

Fig. 3 shows air bubbles that became trapped in an adhesive layer during vacuum lamination of a multilayer stack. The entrapment of the air bubbles in the adhesive layer is undesirable. Thus, there is a need to find a way of eliminating the entrapment of the air bubbles during lamination of the multilayer stack.

As mentioned previously, a smooth round roller is conventionally used to apply constant pressure to the adhesive layer when bonding the light management material to the substrate so that the adhesive layer bonds uniformly.

In contrast, using variable pressure when applying the light management material (i.e. a discontinuous material) to the substrate was surprisingly found to reduce air bubble entrapment. Fig. 4 illustrates an exemplary new variable pressure bonding method to apply light management material 160 to substrate 140 which applies pressure to periodically tack down adhesive prior to lamination. In some cases, heat may also be applied to facilitate bonding.

The exemplary method uses a grooved roller or cog 280 to apply a periodic force to the adhesive as it moves in direction 190 to create first regions 168 having a higher adhesion strength than second regions 169, wherein the second adhesion strength is less than 90% of the first adhesion strength. In an exemplary aspect the first and second regions are disposed transverse or across the width of the light management material 160. In an alternative embodiment, the second adhesion strength is less than 70% of the first adhesion strength. In yet other embodiments, the second adhesion strength is less than 65% of the first adhesion strength. In an exemplary aspect, the overall average peel force is 0.25 N/cm or less, 0.20 N/cm or less or 0.15 N/cm or less.

The grooved roller 280 has a generally cylindrical shape having a plurality of parallel grooves/valleys 282 formed in the outer surface of the roller. The grooved roller further comprises ridges/peaks 284 disposed adjacent grooves. The grooves may be linear grooves, wavy grooves, sawtooth grooves, etc. In a preferred embodiment, the grooves are formed parallel to the central axis 281 of the grooved roller at regular intervals such that the first and second regions are transverse to the travel direction of the roller when the roller is used to apply light management material 160 to substrate 140. In an alternative aspect, the grooves can be formed such that they are biased relative to the central axis in which case the first and second regions are disposed across the width of the light management at an angle relative to the travel direction of the roller. The grooves can be v-shaped, hyperbolically shaped or u-shaped and the ridges may be peaked or rounded.

The first regions 168 can be characterized by a length, L, which is determined by the geometry of the ridges 284 of grooved roller 280. The distance, i, between adjacent first regions defines the second regions 169 and is related to the periodicity of grooves 282. In an exemplary aspect, the transitions between the first and second regions can be sharp or smooth.

The creation of the low adhesion regions (i.e. second regions 169) provides a pathway for air bubbles to be pulled through by vacuum in the first stage of the lamination process while the high adhesion regions (i.e. first regions 168) ensure that the discontinuous material stays in place. In the second step of the lamination process, pressure is applied to the multilayer stack to meld the various layers together while still under vacuum.

While the exemplary roller is described as a grooved roller, one of ordinary skill in the art will recognize that other patterned rollers may also be used so as the second regions provide a substantially continuous pathway. Thus, an exemplary roller can comprise raised posts extending from the surface of the roller wherein the space surrounding the posts will give rise to the low adhesion strength second regions and the tops of the posts will provide the high adhesive strength first regions. The posts can have a circular cross-section, an elliptical cross-section, a circular cross-section, a rectangular cross-section or other polygonal shaped cross-section. In summary, a laminated article can be formed by first creating a multilayer stack of the constituent layers, wherein the layers include a first substrate, a discontinuous structure comprising a member having an adhesive layer disposed on the member, and a flowable material layer. The discontinuous member can be attached to a surface of the substrate by first contacting the adhesive of the discontinuous member to a surface of the substrate and applying pressure to periodically to tack down adhesive layer of the discontinuous member onto the surface of the substrate. A flowable film layer can be laid on the substrate over the discontinuous member.

The multilayer stack van be placed in a vacuum laminator and laminated to create a laminated article.

As mentioned previously, PV modules are laminated structures where the presence of air bubbles in the final structure may be detrimental to one of the reliability or efficiency of the module. The method provided herein is described in additional detail as it pertains to the fabrication of PV modules. First, the multilayer stack is created that includes a backsheet; a light management material, wherein the light management material comprises flexible light redirecting film having a plurality of microstructures extending from a first surface of the film and a continuous adhesive layer disposed on a second surface of the film; an array of PV cells; at least one layer of a flowable encapsulant and a front sheet. The light management material is applied to the backsheet using pressure to periodically tack down adhesive layer of the light management material onto the surface of the backsheet such that the light management material is positioned on the backsheet in the inactive areas of the PV module, e.g. in the gaps between adjacent PV cells and between the PV cells and the edge of the PV module. A first encapsulant film is placed on the backsheet over the light management material and an array of interconnected PV cells is placed on top of the first encapsulant sheet. In an exemplary aspect, the method comprises rolling a grooved (weighted) roller(s) to apply the periodic pressure to the adhesive layer.

An exemplary grooved roller has a surface having a plurality of alternating ridges and grooves formed in the circumferential surface such that the ridges apply greater pressure to the adhesive than the valleys resulting in a bond line between the light management tape and the backsheet has alternating high adhesive strength regions and low adhesive strength regions. In an exemplary embodiment of the grooved roller, the ridges and valleys are disposed parallel to the central axis of the grooved roller and the roller is applied to the light management material such that the ridges and valleys are disposed transverse to a travel direction of the grooved roller. The low adhesive strength regions serve as air migration pathways to allow entrapped air to escape during the vacuum lamination of the multilayer stack. A second film encapsulant sheet is placed over the array of interconnected PV cells and a front sheet placed on top of the second encapsulant sheet.

Next, the multilayer stack is heated to a temperature above the melt temperature of the first and second encapsulant sheets and vacuum is applied to remove entrapped air from the multilayer stack. Finally, a laminating pressure is applied to press the layers of the multilayer stack together completing the PV module lamination process.

In an exemplary aspect, the adhesive layer of the light management material is cured to yield a bond line between the light management tape and the backsheet having a substantially uniform adhesive strength after the lamination process is complete.

While the method describes use of a grooved or textured roller to apply the periodic force to the adhesive, one of ordinary skill in the art will recognize that a periodic force may be applied using a stamping technique with either a one dimensional or two dimensional textured platen or bonding bar. The two dimensional platen can be of approximately the same size as the workpiece or can be smaller than the workpiece and incrementally placed to cover the desired bonding area.

Various embodiments and implementation of the present disclosure are disclosed. The disclosed embodiments are presented for purposes of illustration and not limitation. The implementations described above, and other implementations are within the scope of the following claims. One skilled in the art will appreciate that the present disclosure can be practiced with embodiments and implementations other than those disclosed. Those having skill in the art will appreciate that many changes may be made to the details of the above-described embodiments and implementations without departing from the underlying principles thereof. It should be understood that this disclosure is not intended to be unduly limited by the illustrative embodiments and examples set forth herein and that such examples and embodiments are presented by way of example only with the scope of the disclosure intended to be limited only by the claims set forth herein as follows. Further, various modifications and alterations of the present disclosure will become apparent to those skilled in the art without departing from the spirit and scope of the present disclosure. The scope of the present application should, therefore, be determined only by the following claims.

EXAMPLES

T-peel Adhesion Test Method

Samples were conditioned at room temperature and 50% relative humidity for at least 1 hour prior to testing. One hour before testing the samples were removed from the controlled humidity environment. The T-peel test was used to quantitatively measure the adhesion of the light management material to the backsheet. The T-peel test was done using a MTS Insight 2 from MTS, available from MTS Systems Corporation (Eden Prairie, MN), equipped with a 25N load cell at 12.0 inch/minute (30.48 centimeter/minute) speed. The end of the light management film was separated from the release liner and carefully clamped in the upper jaw of the tester and the free end of backsheet was clamped into the lower jaws of the tester. . Data was reported as Peel strength in Newtons per millimeter (N/mm) and peak load in Newtons (N) at a data sample rate of 10 hz, valley load in Newtons (N), peak load in Newtons (N) and an average peel force in Newtons/mm over a peel range from 0.25 in. to 3.0 in.

For the test specimens created with the grooved roller, the adhesion strength of the first regions (i.e. high adhesion regions) and the second regions (i.e. low adhesion strength regions) can be extracted from the test data by comparing the individual data points to the mean adhesion strength for the specimen. The values above the specimen mean were averaged to give the adhesion strength of the high adhesion strength regions and the values below the specimen mean were averaged to give the adhesion strength of the low adhesion strength regions. From this analysis it is possible to express the adhesion strength of the second regions as a percentage of the adhesion strength of the first regions.

Sample Preparation for Adhesion Testing

A 1.5 in. x 7 in. piece of PET release liner was placed along the log edge of a 5 in. x 7 in. solar backsheet TCP, available from Lucky Film Company (China), with approximately a 0.75 in. overlap such that the release coated side of release liner is disposed along the top of matte side of backsheet. A piece of 3M™ Polyester Tape 8402 available from 3M Company (St. Paul, MN) was applied to the backside to temporarily hold the release liner and backsheet in the desired configuration.

The backsheet with attached release liner was heated for approximately 20-30 seconds at a temperature of 100°C on the bed of an NPC Photovoltaic Module Laminator, Model LM-110 X 160-3, available from NPC Incorporated (Tokyo, Japan) for about 20-30 seconds to remove any curvature.

With the prepared backsheet disposed on the bed of the laminator, a 4 in. segment of light management material, such as a light redirecting film available from 3M Company (St. Paul, MN) was applied by first placing one end of the LRF material on the release liner such that it overlapped the release liner by 0.5 in. Thumb pressure was applied to the light management material to tack it in place, making sure to avoid contacting the remaining portion of the light management strip from contacting the backsheet. Next, the LRF was attached to the backsheet according to the process described in Table 1.

Table 1.

Creating a Pseudo-PV module

5 pieces of BC81 light redirecting film (LRF) were cut to 7 inches long. An 8 in. x 8 in. x0.125 in. piece of glass was cleaned with acetone. Two pieces of polyolefin encapsulant (POE)

(TP4 from Hangzhou First Applied Material Co. Ltd, China) and one piece of transparent solar backsheet TCP, available from Lucky Film Company (China), were cut to 8 in. x 8 in. The backsheet was cleaned with a tissue to remove any foreign debris.

The backsheet was placed smooth side down on the bed of the laminator which had been preheated to 100°C for about one minute. One piece of the BC81 light redirecting film was placed on the rough upward facing surface of the backsheet such that the LRF was positioned 1 in. from the edge of the back sheet and 0.5 in. from the side of the back sheet perpendicular to the edge. The LRF was pressed down onto the backsheet with thumb pressure for about 5 seconds to tack down the end. Next, the applicator, as indicated in Table 2, was rolled along the length of the LRF taking about 5 seconds to traverse the length. The remaining piece of LRF were placed down in a similar fashion at a spacing of 1.5 in. away from the previously placed segment of LRF. The backsheet was then removed from the laminator and allowed to cool.

Table 2

Next, two pieces of POE were placed onto the back sheet over the strips of LRF and the glass was placed on top of the encapsulant, completing the multilayer stack for the pseudo PV module test vehicle. PET tape was placed periodically around the perimeter of the multilayer stack to facilitate handling and lamination.

Two polytetrafluoroethylene (PTFE) sheets were placed onto the bed of the laminator and the laminator was heated to 128°C. After the laminator reached temperature, the multilayer stack was placed between the PTFE sheets, glass side down. The lamination cycle was started with an eight-minute vacuum period, during which no pressure was applied, and all air was pulled from the chamber. Next, 60kPa pressure was applied to the multilayer stack. After 2 minutes, the laminator temperature was increased to 145°C. When the laminator reached, temperature, the 60kPa pressure was maintained for an additional 10 minutes. After the cycle was complete, the panel was removed from the laminator; the PTFE sheets were peeled off; and the panel was allowed to cool.

Visual inspection was done on the panels before and after lamination, noting the presence or absence of air bubbles in the adhesive bonding the LRF to the backsheet.

Fig. 5 A and 5B show a portion of the pseudo PV module of example Clp created using a smooth wooden roller to apply the LRF to the back sheet before and after lamination, respectively. Fig. 5B shows evidence of entrapped air bubbles in the adhesive layer between the LRF and the backsheet after lamination.

Fig. 6 A and 6B show a portion of the pseudo PV module of example Exlp before and after lamination, respectively. Prior to lamination, the adhesive appears to have a slight lateral pattern due to the ridges in the groove roller exerting greater force on the adhesive layer during roll down (i.e. first regions) than the areas corresponding to the roller’s grooves (i.e. second regions) as shown in Fig 6A. No air bubbles became entrapped during lamination as shown in Fig. 6B.

Claims

What is Claimed is:

1. A method of applying an adhesive to a substrate to mitigate entrapment of air bubbles during a vacuum lamination process, the method comprising: contacting an adhesive to a surface of the substrate; applying a periodic force to the adhesive along the longitudinal direction creating first regions having a first adhesion strength to the substrate and second regions having a second adhesion strength to the substrate, wherein the second adhesion strength is less than 90% of the first adhesion strength.

2. The method according to claim 1, wherein the adhesive comprises a smooth, generally planar layer formed on a support layer.

3. The method according to either of claims 1 or 2, wherein the periodic force is applied by a grooved roller having a plurality of parallel grooves formed in an outer surface of the roller.

4. The method according to claim 3, wherein each of the plurality of grooves is parallel to a central axis of the roller.

5. The method according to either of claims 3 or 4, wherein the roller further comprises ridges disposed adjacent grooves, wherein the ridges apply a higher force than the grooves in the roller when the roller applies pressure to the adhesive.

6. The method according to any of claim 3-5, wherein the ridges and/or grooves are disposed transverse to a direction of movement of the grooved roller relative to the adhesive.

7. The method according to any of the proceeding claims, wherein the adhesive is a thermally activatable adhesive.

8. The method of claim 7, wherein the thermally activatable adhesive is a hot melt adhesive.

9. The method of either of claims 7 or 8, wherein the thermally activatable adhesive is a thermally activated cross-linkable adhesive.

10. A method of fabricating a photovoltaic module, comprising: creating a multilayer stack comprising a backsheet and a light management tape, wherein the light management tape comprises flexible light redirecting film having a plurality of microstructures extending from a first surface of the film and a continuous adhesive layer disposed on a second surface of the film; contacting the adhesive to a surface of the backsheet; applying pressure to periodically tack down adhesive layer of the light management tape onto the surface of the backsheet; laying a first film encapsulant sheet over the backsheet; placing an array of interconnected solar cells on top of the first encapsulant sheet; laying a second film encapsulant sheet over the array of solar cells and a front sheet; and vacuum laminating the multilayer stack to create the solar module.

11. The method of claim 10, wherein the step of vacuum laminating the multilayer stack comprises: heating the stack to a temperature above the melt temperature of the first and second encapsulant sheets; applying vacuum to remove entrapped air from the multilayer stack and applying pressure to the multilayer stack; and applying a lamination pressure to press the layers of the multilayer stack together.

12. The method of claim 11, wherein the step of applying pressure to adhesive layer comprises rolling a grooved roller along the length of the light management tape.

13. The method of claim 11, wherein the grooved roller has a surface having a plurality of alternating ridges and valley formed in the surface such that the ridges apply greater pressure to the adhesive than the valleys such that a bond line between the light management tape and the backsheet has alternating high adhesive strength regions and low adhesive strength regions.

14. The method according to either of claims 12 or 13, wherein the ridges and/or grooves are disposed transverse to a direction of movement of the grooved roller relative to the adhesive.

15. The method of either of claims 13 or 14, wherein the low adhesive strength regions serve as air migration pathways to allow entrapped air to escape during the vacuum lamination of the multilayer stack.

16. The method of any of the previous claims, further comprising curing the adhesive layer of the light management tape to yield a bond line between the light management tape and the backsheet, wherein the bond line has a substantially uniform adhesive strength.

17. A vacuum lamination method, comprising: creating a multilayer stack comprising a first substrate, a discontinuous structure comprising a member having an adhesive layer disposed on the member, and a flowable material layer comprising the steps of contacting the adhesive of the discontinuous structure to a surface of the substrate; applying pressure to periodically tack down adhesive layer of the member onto the surface of the substrate; laying a flowable film layer over the substrate; and vacuum laminating the multilayer stack to create a laminated article.