WO2019014720A1 - A method for fabricating a photovoltaic module - Google Patents

Abstract

The present disclosure provides a method of forming a conductive metal pattern either within or on a surface of a polymeric material. The method comprises providing the polymeric material and forming a predetermined pattern on or within the polymeric material. The method also comprises forming a conductive metal pattern aligned with the predetermined pattern using an electrochemical process and applying a bonding material to at least a portion of the conductive formed conductive metal pattern. The bonding material is suitable for bonding to conductive regions of electrical devices, such as solar cells, and can thereby facilitate the interconnection of an array of solar cells into a photovoltaic module.

Description

A METHOD FOR FABRICATING A PHOTOVOLTAIC MODULE

Technical Field of the Invention

The present invention relates to the field of photovoltaic module fabrication and, particularly but not exclusively, to the interconnection of individual solar cell devices to form a photovoltaic module that can be connected into an electrical circuit to generate electrical power.

Background of the Invention

Most photovoltaic modules that are used to convert sunlight into electricity comprise of arrays of silicon (Si) solar cell devices . The individual solar cells are typically fabricated using doped crystalline Si wafers which are processed to include electron-collecting and hole collecting regions . These carrier collecting regions can be formed by solid state diffusion or by the application of surface layers that can selectively collect one carrier polarity when the cell is illuminated. In order to extract the photo-generated electrical carriers from the solar cell, conductive regions comprising metal electrodes (or contacts) are typically formed in contact with both the electron and hole-contacting regions of the cell. Conducting wires can then be bonded or connected to these metal electrodes enabling the individual solar cells to be connected into arrays of solar cells configured either in a series or parallel electrical arrangement. The thus-laid out and interconnected cells can then be laminated with encapsulant into a photovoltaic module that can be installed in the field to generate electricity through the absorption of solar energy.

The solar cells of most Si photovoltaic modules are

interconnected by soldering tabbing wire to metal busbar regions formed on each of the solar cells in a photovoltaic module.

These busbar regions are typically in contact with arrays of thinner metal tracks, or fingers, which are oriented

perpendicular to the busbars and act to extract the photo- generated current from the underlying silicon. The electrode grid comprising conductive fingers and busbars comprises the cell-level metallization and is typically formed by screen- printing a metal paste containing silver or aluminium on the solar cells. The grid electrode can also be formed by plating metal such as nickel, copper (Cu) and/or Ag . Although, the latter process can result in reduced metal material costs and very narrow metal fingers, it is less commonly used due to being a less established process.

The soldering process requires localized heating and pressure which can result in microcracks in the Si wafers. This is a particular concern when thin Si wafers (e.g., < 160 μιτι) are used in order to reduce the Si material cost. Another undesirable aspect of this interconnection process is that typically the conductive tabbing wire used to interconnect adjacent cells in a module is coated in a lead (Pb) -based alloy to ensure a low melting point and reduce the temperature at which the soldering can be performed. It is undesirable to use lead in manufactured products due to its deleterious effect on humans and other species. Lead-free solders can be alternatively used; however, these alloys typically require soldering at higher temperatures which is undesirable.

Although soldering busbar interconnections have been an industrially-accepted way of interconnecting solar cells for many years, the increased photo-generated current achieved in more recently manufactured cells due to their larger areas and improved energy conversion efficiencies has made it more challenging to reduce power losses due to series resistance. This problem can be addressed by increasing the number of busbars on the cell. Through this adaptation, power losses due to series resistance can be reduced as the magnitudes of the current flowing in both the fingers and busbars is reduced from that in solar cells with only two or three busbars. Furthermore, cell metallization costs can be reduced as fingers can be thinner and therefore less Ag is required. However, these performance and cost advantages are offset by increased complexity of the automated tabbing processes due to the stricter requirements for alignment of thinner busbars.

Furthermore, adhesion of the busbars can be impacted by the reduced cell contact area for each individual busbar.

To address these challenges a number of alternative

interconnection methods have been developed which do not require busbar contact regions to be formed on the solar cell surfaces. In one alternative approach, an array of Cu wires, coated in a low melting point alloy, are aligned in an array and adhered to an adhesive polymer. This wire-containing polymer sheet can then be placed over the metallized cells, with bonding occurring between the metal fingers of the solar cell and the alloy-coated wires during the lamination process. This proprietary process, which was first reported by Day-4 Energy, is being commercialized by Meyer Burger as their Smartwire

Interconnection Technology (SWCT) .

An alternative wire-based interconnection process is the MultiBB process first reported by the German Company Schmid. Unlike the SWCT process, the MultiBB process does not require the Cu wires to be held in an adhesive polymer. Instead tabbing equipment is provided that positions the wires over arrays of metallized cells and the bonds between the wires and the cells are achieved by infrared soldering. Other variations on these wire-based interconnection schemes have been reported. For example, in GTAT' s Merlin technology, a free-form metal wire network is placed over the cell surface with bonding occurring during the lamination process. Meyer et al . reported a multiwire stringing approach which uses a perpendicularly-arranged wire network running between an upper and lower spooled wire array. In the latter case bonding is achieved using soldering and mechanical actuators are employed to cut the wires to prevent electrical shorting (L. Meyer et al . , Manufacturable Multiwire Stringing and Cell Interconnection for Si Cells and Modules, IEEE

Photovoltaics Specialist Conference 2017, Washington, DC) .

In all these alternative wire-based interconnection processes, the need to form busbar contact regions on the cells is eliminated. This is beneficial for solar cell device performance as the rate of electrical carrier recombination at metal contact regions is much higher than at surfaces coated with a dielectric material. Furthermore, by being able to position the wires more closely than is possible with existing tabbing wire (which ranges from 0.8 to 1.5 mm in width), the length of the fingers (i.e., distance between busbars) can be substantially reduced. This feature of wire-based interconnection is advantageous for both screen-printed and Cu-plated metallization. For screen- printed cell metallization, it can reduce the required thickness of the paste thereby reducing the amount and hence cost of Ag required. For plated cell metallization, less metal needs to be plated which can improve the adhesion of the metal and reduce the plating time. Furthermore, the use of circular wires in place of flat tabs of interconnection wire can result in increased trapping of light in the underlying cells of a module as rays impinging on the curved surface are reflected back to the front glass at a more oblique angle resulting in a higher probability of the rays being totally internally reflected back into the cell thereby increasing the probability of being absorbed in the solar cell resulting in increased photocurrent . However, these alternative processes can also present some disadvantages. Typically they result in an increased

interconnection cost to Si photovoltaic module producers compared to the more traditionally-used soldering of tabbing wire. These costs can arise due to the use of indium-containing metal alloys and the adhesive polymer sheets (in the case of SWCT), and expensive proprietary tabbing equipment or mechanical actuators required to perform the interconnection processes at the manufacturers' factories.

Part of the complexity required by the wire processing equipment arises due to the use of continuous preformed wires . For series- interconnection of the most commonly-produced solar cells which have electrodes on both surfaces, the wires must extend from the p-type surface of one cell to the n-type surface of the adjacent cell (i.e., from the front to rear surface of cells) . The tabbing equipment must therefore include either wire cutting and bonding functionality or complex cell layout capability.

The use of wires also limits the flexibility of the

interconnection arrangement. Historically photovoltaic modules have used bypass diodes to mitigate the impact of localized shading by providing an alternative current pathway in parallel to the cell string that can prevent power being dissipated in non-shaded cells and causing localized heating. With the reduced cost of module-based electronics, string DC/DC optimisers and different series/parallel configurations can be used as alternatives to bypass diodes. If the string-level DC/DC optimisers can be integrated into the module in a way that allows them to be replaced on failure then this can provide benefits over the case where bypass diodes are integrated within the encapsulated photovoltaic module. There is therefore an opportunity to design interconnection circuitry in such a way that bypass diodes can be dispensed with in favour of more flexible, replaceable on failure, module-level circuitry.

The high costs of wire-based interconnection schemes that rely on metal alloy bonding during lamination arise in part because the entire surface of the alloy-coated wires must be coated with the metal alloy material, although practically, alloy is only required on the surface of the wire that bonds to the metal fingers. This factor results in increased material costs, which can be considerable if expensive metals (e.g., indium) must be used to achieve reliable bonding to the metal fingers on the solar cell. What is required is an improved solar cell interconnection method.

Summary of the Invention

Aspects of the present invention provide techniques that allow interconnecting cells without using traditional busbar regions. This allows to provide the benefits of wire-based

interconnection methods and, at the same time, provides a lower- cost and more flexible solution for module manufacturers.

In accordance with a first aspect the present invention provides a method of forming a conductive metal pattern on at least a portion of a polymeric material, the method comprising the steps of:

providing the polymeric material;

forming a predetermined pattern on or within the polymeric material ;

forming a conductive metal pattern aligned with the predetermined pattern by depositing a first material using an electrochemical process; and

applying a second material to at least a portion of the formed conductive metal pattern, the second material being suitable for bonding to at least the portion of the formed conductive metal pattern and to one or more conductive surfaces of an electrical device.

In one specific embodiment the method comprises:

providing at least one electrical device having one or more conductive surfaces ;

contacting the one or more conductive surfaces of the at least one electrical device with the second material applied to the at least a portion of the formed conductive metal pattern; and

annealing the polymeric material with the one or more conductive surfaces of the at least one electrical device such that the applied second material bonds to the one or more conductive surfaces of the at least one electrical device and the at least a portion of the formed conductive metal pattern. Annealing the polymeric material is typically conducted at a temperature up to 160°C. The annealing is typically conducted in an environment in which an ambient pressure is 40-60kPa.

The electrical devices may comprise one or more of: a

semiconducting device such as a solar cell and light-emitting diode, an electrical circuit containing passive and active elements, and a sensor or an antenna element.

The method may further comprise lining at least a portion of the polymeric material at the formed predetermined pattern with a third material prior to forming the conductive metal pattern aligned with the predetermined pattern. The third material may be a refractory metal or a metal alloy comprising one or more elements from the group of Ti, Ta, W, Mo, Ni, Pt, N, Co and Si.

Forming a predetermined pattern on or within the polymeric material may comprise:

applying a masking layer to a surface of the polymeric material; and

forming the predetermined pattern within the masking layer and polymeric material.

Further, lining a surface of the polymeric material at the predetermined pattern may comprise:

depositing the third material over the predetermined pattern that has been formed and the polymeric material; and

removing the masking layer such that the third material only covers the polymeric material at the predetermined pattern .

The masking layer may be attached to the polymeric material by application of heat and/or pressure.

The predetermined pattern may include at least one recess or hole that penetrates through the entire thickness of the polymer. The metal of the conductive metal pattern may penetrate through or into the at least one recess and/or into the at least one hole.

The third material may be arranged to provide a diffusion barrier for the first material preventing or reducing the likelihood of diffusion of the first material into the polymeric material .

Alternatively or additionally, the third material may be arranged to provide a seed layer facilitating the adhesion of the first material when it is deposited on the surface of the third material.

In embodiments, the material suitable for bonding to metal of the conductive metal pattern comprises a low-melting point metal alloy .

In some embodiments, at least the portion of the conductive metal pattern and the applied second material may be heated before the annealing to reflow the applied second material to form a uniform alloy coating over at least a portion of the conductive metal pattern.

Reflowing may comprise applying localized heat to form the uniform metal alloy coating over at least a portion of the conductive metal pattern.

In some instances, reflowing is performed at a temperature between 160°C and 180°C. Alternatively, reflowing may be performed at a temperature between 140°C and 160°C.

In embodiments, the second material comprises an electrically conductive adhesive.

In embodiments, forming the predetermined pattern aligned with the surface of the polymeric material comprises:

structuring one or more regions of the surface of the polymeric material; and

selectively depositing the third material only on the areas of the surface of the polymeric material that are structured .

The structuring may be performed using a pulsed laser having a pulse duration of less than 15 ps . Further, the structuring may be performed using a UV laser.

The third material may be deposited using an electroless deposition process and can be one of the following materials: Ni, Sn, Ag, W, Mo, Co or alloys of these elements.

In some embodiments, the structuring is performed such that an opening is formed and which extends through the thickness of the polymeric material. The conductive metal pattern may be formed by electroplating and may extend throughout the thickness of the polymeric layer.

Further, lining a surface of the polymeric material at the formed predetermined pattern with a third material may comprise sputtering a metal on the surface of the polymeric material or a masking layer to form a conductive seed layer.

In some embodiments, the predetermined pattern on or within the polymeric material is formed using one of, or a combination of, inkjet printing, aerosol printing or screen printing to deposit a conductive seed layer.

In some embodiments, at least a portion of the formed conductive metal pattern is formed to increase scattering of light incident on the conductive pattern surface. The light scattering of the conductive metal pattern may be achieved by using an

electroplating mandrel which has a structured surface.

In embodiments, forming the conductive metal pattern includes an electroplating step. The electroplating step may be performed using a reel-to-reel process in which the polymeric material passes through an electrolyte comprising at least one metal ion salt .

In some embodiments, the conductive metal pattern contains Cu . Further, the material capable of bonding to a metal of the conductive metal pattern may be applied through either a printing or dispensing process.

In some instances, the low melting-point metal alloy is formed by an electroplating process involving a chemical bath

comprising metal ions corresponding to the elements of the low melting-point metal alloy.

In accordance with the second aspect, the present invention provides a method of fabricating a photovoltaic module

comprising at least two solar cells, with at least two of the solar cells having one or more conductive regions on at least one surface, the method comprising the steps of:

providing a module back sheet, the back sheet comprising either a polymer, polymer composite or glass material;

providing a first sheet of a polymeric material, the polymeric material having a predetermined conductive metal pattern with at least a portion coated with a bonding surface comprising of a second material exposed on a surface of the polymeric material;

arranging the first sheet of the polymeric material over the module back sheet;

arranging the plurality of solar cells over the first sheet of the polymeric material, each solar cell having at least one conductive region on a surface of the solar cell, and the solar cells being arranged such that the at least one conductive region from each solar cell is in contact with the said bonding surface exposed on the first sheet of the polymeric material; providing a second sheet of a polymeric material arranged to cover the plurality of solar cells;

arranging a substantially transparent front sheet over the second sheet of the polymeric material; and

laminating the module back sheet, the first sheet of the polymeric material, the plurality of solar cells, the second sheet of the polymeric material and the front sheet;

wherein, during lamination, at least one conductive region of the plurality of solar cells bonds with a portion of the bonding surface exposed on the first sheet of the polymeric material and an electrical circuit connection is formed between at least one pair of adjacent solar cells.

The back sheet me be formed from any suitable material including suitable glass or polymeric materials.

The first sheet of the polymeric material may be fabricated in accordance with the method of the first aspect of the present invention .

In accordance with the third aspect, the present invention provides a photovoltaic module fabricated in accordance with the method of the second aspect.

The electrical connection between two adjacent solar cells in the photovoltaic module may be formed by an element of the conductive metal pattern in the polymeric material that extends between the two adjacent solar cells.

The electrical connection between adjacent solar cells in the photovoltaic module may be formed by bonding between aligned bonding surfaces exposed on both the first and second sheets of polymeric material, said bonding occurring during the laminated step.

The conductive regions on the solar cells may comprise metal elements. The conductive regions of the solar cells may also comprise conducting oxide regions. Advantages of embodiments of the present invention include a new methodology for efficiently interconnecting cells in a

photovoltaic module. In particular, in accordance with

embodiments, cells can be interconnected without using

traditional busbar regions. Advantageously, this reduces cost while preserving or improving the electrical performance of the devices . Further, advantages are provided for photovoltaic module producers that do not have to purchase and maintain expensive proprietary tabbing equipment to achieve performing cell interconnection.

Brief Description of the Drawings

The embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which :

Figure 1 is a schematic diagram that depicts the individual layers comprising a bifacial photovoltaic module fabricated using a method in accordance with embodiments;

Figure 2 is a flow diagram with steps used to fabricate a metallized polymer sheet in accordance with embodiments;

Figure 3 shows flow diagrams with steps used to fabricate metallized polymer surfaces in accordance with embodiments;

Figure 4 shows different arrangements of a metallized element on a polymer surface or within a polymer in accordance with embodiments;

Figure 5, 6 and 9 show the metallization process for a sheet of a polymeric material at different stages for different variations of the process;

Figure 7A shows an example of a metallized polymer sheet (after Cu plating) that has been cut to the size of a 156 mm solar cell;

Figure 7B shows an example of a metallized polymer sheet with a conductive element that has been capped with a bonding material comprising a tin-bismuth-silver alloy paste;

Figure 8 is an illustration of electroplating equipment configuration in which the polymer can be separated from the structured electrode on completion of the plating process;

Figure 10 is a flow diagram with steps used to fabricate a photovoltaic module using at least one metallized encapsulating polymer sheet in accordance with embodiments; Figure 11 is a cross-sectional schematic of a bifacial photovoltaic module fabricated using the method shown in Figure 10; and

Figure 12 shows an alternative module arrangement where groups of shingled solar cells are interconnected into subassemblies using a metallized encapsulating polymer sheet.

Detailed Description of Embodiments

A method for interconnecting solar cells into a module is described below. Although described with reference to Si photovoltaic module fabrication, the method can also be applied to photovoltaic modules comprising other absorber materials and other electrical and electronic devices requiring electrical interconnection and/or encapsulation.

Embodiments of the method allow to simplify the cell

metallization and interconnection process for photovoltaic module producers and eliminate the need for wire handling processes to be performed as part of the product manufacturing line .

Embodiments of the described method allow to simplify the manufacturing process by making it potentially possible for photovoltaic module producers to purchase pre-metallized encapsulant (polymer) sheets and perform cell interconnection as part of the module lamination process. Like the existing wire- based interconnection processes, embodiments of present invention do not require busbar regions to be formed on the cell surface enabling higher cell voltages to be achieved. However, embodiments of the present invention provide the additional benefits of reduced material cost, greater flexibility in metallization pattern due to the direct metallization process and enhanced light trapping through the incorporation of light structuring features in the conductive metal pattern formed directly on or in the encapsulant at the surface.

The method involves producing a conductive metal pattern on a polymer (which is used as an encapsulant in the module) and applying a bonding material to at least a part of the conductive metal pattern such that a portion of the conductive metal pattern is exposed. The polymer may have through-holes (also called vias) and/or recesses and/or flat surface portions. The conductive material may at least partialy penetrate through the through-holes and/or penetrate at least partially into the recesses and/or extend along flat surface portions of the polymer. When the thus-metallized polymer is heated and the exposed portion of the conductive metal pattern is in contact with a second conductive surface (e.g., the metal finger and busbar regions of a metallized solar cell), a bond forms between the conductive metal pattern of the metalized polymer and the conductive surface of the electrical or electronic device.

Two of such metallized polymer sheets may be used to fabricate a photovoltaic module comprising an array of individual solar cells where current is extracted from both surfaces of the cell. The conductive metal patterns of each metallized polymer sheet can be designed such that a photovoltaic module is fabricated by simply laying cells over a first metallized encapsulating polymer sheet placed directly on a module back sheet (or glass), covering the arranged cell layer with a second (upper)

metallized encapsulating polymer sheet and module glass, and then laminating the assembly. During the lamination step, exposed portions of the conductive metal pattern of the metallized polymer bond with regions of the cell metallization through the action of the bonding material. Furthermore, bonding between metal regions on the first and second polymer sheets interconnect adjacent solar cells in the module allowing current to flow between them. As the polymer softens on heating, it encases the interconnected solar cells as an encapsulant. The bonding material can comprise a metal alloy that melts at a temperature less than the lamination temperature (i.e., below 160°C) or an electrically-conductive adhesive (ECA) .

Figure 1 schematically depicts the individual layers comprising a bifacial photovoltaic module 100 fabricated using the cell interconnection and module fabrication process. Bifacial modules can provide a number of benefits due to their ability to convert light from both surfaces of the solar cell into electricity. They provide advantages in highly reflecting environments as light that is incident on the ground or background can be reflected into the modules via their rear surface resulting in an energy conversion efficiency which is enhanced by an albedo factor which can be as large as 30% for highly reflecting surfaces .

The photovoltaic module assembly 100 shown in Figure 1 comprises two adjacent bifacial solar cells 130 having linear metal fingers 132 and 134 arranged on the two surfaces of the cells, respectively. The metal fingers can be formed through the screen-printing of metal pastes such as Ag over the surface dielectric layer that acts as an antireflection coating (ARC) . The solar cells are typically 'fired' by being passed through a high temperature belt furnace where they experience temperatures in the range of 700°C to 850°C for several seconds. This process causes the screen-printed metal to penetrate through the ARC and make intimate contact with the underlying Si of the solar cell.

Alternatively, the metal fingers on the solar cells can be formed by first ablating thin linear tracks in the ARC using a laser and then plating the exposed Si of contact layer with a metal stack, preferably comprising layers of Ni , Cu and a capping layer of Ag or Sn. The plating can be achieved by electroless plating, by using the light-induced current of the solar cell or by providing an external bias to the other surface of the cell such that any semiconducting junctions in the solar cell are forward-biased.

The capping layer on Cu plated cells should be sufficiently thick to enable a reliable bond (e.g., metal-metal alloy) bond during lamination. The thickness of Ag capping layers in an embodiment of the present invention should have a thickness of at least 80 nm and preferably thicker than 100 nm to provide a sufficient coating for the underlying Cu and enable a reliable bond with the bonding material on the metallized encapsulant. Thicker capping layers can be used, but are generally

undesirable due to the high cost of Ag . When using Sn as a capping layer, preferably a layer thickness of greater than 1 μπι is formed and more preferably layers thicker than 2 μπι. The capping layers can be applied using the plating processes described above for the Ni and Cu . Alternatively thinner capping layers can be formed by immersion or displacement plating where a surface layer of Cu is electrochemically displaced with a thin layer of the capping metal.

The spacing and thickness of the metal fingers 132 and 134 on the solar cells can be tuned to limit power losses due to series resistance. This optimization process, which depends on the conductivity of the underlying cell contact layer, the

resistivity of the metal used, the cross-sectional area of the fingers and the distance the collected current must travel before it is collected in the conductive tracks (pattern) of the metallized encapsulant, is documented for example in "Solar Cells: Operating Principles, Technology, and System

Applications" by Martin Green. In configurations, such as that depicted in Figure 1, where the conductive metal tracks embedded in the encapsulant are equally spaced and oriented perpendicular to the fingers, the distance that current must flow along a finger before being transferred into the conductive tracks is equal to half the spacing between the conductive tracks.

Depending on the different conductivities of the electron and hole collecting layers of the solar cell, the spacing of the fingers can differ on the two surfaces of the solar cell.

The solar cells are arranged on a first metallized encapsulating polymer sheet 120, which is placed on the module back sheet 110, and contains a conductive metal pattern formed on at least one surface (upward facing in Figure 1) . In the example depicted in Figure 1, the conductive metal pattern comprises an array of linear metal tracks 125 that are aligned perpendicular to the metal fingers 132 on the solar cells 130 and connected to an interconnection tab 128 which is also formed on the metallized encapsulating polymer sheet. A second metallized encapsulating polymer sheet 140 with a corresponding conductive metal pattern comprising an array of linear metal tracks conductive tracks 145 and an interconnection tab 148 is then placed over the arranged cells such that the exposed conductive metal pattern is facing downwards in Figure 1 and the cell interconnection tabs 128 and 148 are aligned and can bond during lamination.

The assembly is then laminated with a glass or polymer front sheet 150. During lamination bonds form between the metal elements 145 and 125 that were formed on the encapsulating polymer and the respective metal fingers 134 and 132 on the solar cells 130. Additionally bonds form between the aligned interconnection tabs 128 and 148. Bonding between the

interconnection tabs 128 and 148 on the first and second encapsulating polymer sheets 120 and 140, respectively, can be achieved through melting of a metal alloy or bonding through the application of an ECA to the bonding sites.

Although depicted in Figure 1 as continuous linear elements, the interconnection tabs 128 and 148 can alternatively comprise an array of tabs. The lamination process which is performed at a temperature of between 130 and 170 °C and more preferably at 150°C for 8 to 15 minutes and more preferably for 10 min, also acts to encapsulate the module, sealing it from moisture ingress during field operation. After lamination, a frame and junction box are added to the assembly to complete the fabrication of the module. This module fabrication process can be performed by a photovoltaic module producer simply by purchasing pre-fabricated metallized encapsulant sheets and performing the module layout process using automated cell placement technology. Such placement technology can accurately place individual solar cells to within 10 μπι of a target location.

Although the fabrication of a bifacial cell is depicted in Figure 1, the same process can be used to interconnect cells with fully-metallized rear surfaces provided that the cell surfaces can bond sufficiently with the bonding material on the encapsulant surface. For example, if the rear surfaces of the solar cells comprise screen-printed and ^>fired' Al, then the surface may need to contain bonding regions comprising another metal such as Ag or be pre-treated before bonding can be achieved. Options for this pre-treatment include zincating and use of the tin-pad technology commercialized by Schmid.

Another option made possible by the pattern flexibility and the high conductivity of the rear Al electrode is to have a different conductive metal pattern (e.g., fewer conducting elements) on the front and rear surfaces of the cell. Since the rear Al electrode is very conductive and non-transparent then fewer wider conductive elements can be used in the metallized polymer with bonding achieved using an ECA.

It should also be clear from the above description that the interconnection process can also be applied to interdigitated back contact (IBC) cells. For example, it could be used to achieved cell interconnection method for IBC cells metallized as described by Z. Li et al in "Electrical and optical analysis of polymer rear insulation layers for back contact cells"

(published in Energy Procedia, 77, 744-751) and U. Romer et al, in "Decoupling the metal layer of back contact solar cells - optical and electrical benefits" (published in Energy Procedia, 77, 744-751) .

A key advantage of this process over existing wire-based interconnection processes is the increased flexibility of the metallization pattern. Although the process is described below with respect to the interconnection of bifacial solar cells through a series of linear metal elements formed directly on the encapsulant, the direct patterning of the polymer's surface allows for alternative patterns including non-linear patterns to be formed.

The method for fabricating the metallized polymer sheets used to interconnect and encapsulate solar cells in a module will now be described with reference to Figure 2 and Figure 3. The metallized polymer sheets produced through this process can also be used for other applications, including applications where the polymer is not acting as an encapsulant. The process 200 in Figure 2 summarises the general fabrication process for the metallized polymer sheets. In step 210, a pattern is produced either within the polymer or on the polymer's surface. The pattern can be represented digitally and produced on the polymer using a laser, a digitally-controlled printer (e.g., inkjet or aerosol printer) or can be mediated through a mask or a screen for a sputtering or screen-printing process, respectively. In step 220, a conductive metal pattern is then formed either within the polymer or on the polymer's surface aligned to the pattern formed in step 210. This step is preferably performed by electroplating, also called electroforming . The conductive metal (e.g., Cu) is provided as metal ions (e.g., Cu sulphate, Cu nitrate) supported in an electrolyte which can be aqueous, organic or comprise a solid electrolyte or ionic liquid. The electroplating step can be performed in continuous fashion using a reel-to-reel (R2R) process where the patterned polymer is fed from one reel (roll) through an electrolyte comprising metal ions whilst in contact with an electrical potential and then collected on a second reel. Different approaches can be used to provide the electrical current for the electroplating step and these are described below for different arrangements.

The metal used to form the conductive metal pattern is

preferably Cu due to its high conductivity, its ductility and its ability to be electroplated at high current densities. As will be described further below, a thin barrier layer is preferably formed on the polymer to prevent diffusion of material from the conductive metal (e.g. Cu) into the polymeric material. Finally, in step 230, bonding material is applied to at least a region of the conductive metal pattern surface to enable bonding to the conductive surfaces of one or more electrical or electronic devices (e.g., solar cells) and a second metallized polymer surface for bifacially interconnected devices .

This polymer metallization process can be applied to a range of encapsulants , though preferably highly water resistant materials such as thermoplastic polyolefins (TPO), silicones (such as Dow Corning Sylgard 184) or iononer-based encapsulants such as DuPont's PV5400 and PV8400, or ethylene vinyl acetate (EVA) are used. An important property of polymers used as encapsulants is that they are highly resistant to ion migration under potential stress so as to ensure that resulting modules do not experience potential-induced degradation. Additionally it is important that the polymer used does not result in corrosive byproducts on aging. For example, EVA materials that result in low levels of acetic acid are preferable. If EVA is used, then it is important that moisture ingress is prevented as much as possible by either using glass as the back sheet or using a highly water resistant back sheet. Additionally, it is necessary to ensure that the EVA used does not contain stabilizing additives that can react with Cu in photo-oxidation reactions .

The process can also be applied to polymers not specifically used for device encapsulation and where the metal pattern comprises parts of an electrical/electronic circuit or sensor or antennae elements. For this reason, the encapsulating polymer will simply be referred to in the following description as a polymer' to indicate this generality.

The different methods of metallizing the polymer are now described with reference to the flow diagrams in Figures 3A, 3B and 3C. Figure 3A details the steps of process 300 in which a laser is used to ablate a pattern in the surface of the polymer which can be selectively plated. An advantage provided by producing the pattern using laser structuring is that it allows for the formation of a structured metal surface that can be used to enhance the light trapping within the module thereby increasing the module's current collection efficiency. The roughness induced by the laser can also enhance the adhesion of the subsequently formed conductors.

In step 305 a laser is used to ablate a pattern in the polymer. The depth of the ablated regions can be adjusted by using different laser wavelengths, fluences and number of passes. In one arrangement a UV laser with a wavelength below 400 nm (e.g., 266 or 355 nm) is used as polymers absorb light more strongly in this wavelength region. Additionally, if the laser has a pulse duration in the range of several femtoseconds to ~ 10

picoseconds, then the ablated surface is typically rougher due to non-linear absorption in the polymer. This polymer roughness can induce a conforming metal roughness, or structure, when the ablated regions are metallised. Figure 4A shows a schematic of a polymer 400 with a laser-ablated region 405 filled with a conductive metal 410 and capped with a bonding material 415. This schematic shows how the conductive metal filling the laser- ablated opening assumed the structured (rough) surface

introduced by the laser ablation process.

Alternatively, light structuring surfaces can be deliberately introduced into the laser ablation pattern as shown by the gratings 420 in Figure 4B. In this case because a microstructure is introduced by a specific pattern, it is not as critical to use a short pulse laser and a larger range of lasers can be used, including lasers with nanosecond pulse durations and lasers with longer wavelengths, such as C0₂ lasers.

The method used for structuring the polymer surface can be guided by optical simulations that use the optical modelling approach published by Y. Li et al . (IEEE Journal of

Photovoltaics, 4 (5), 1212-1219, 2014, Optics Express, 23 (24), p. A170) to integrated modelling of nanoscale and microscale light trapping features within the multi-layered final module. The optical effects of the different laser structuring (i.e., random features as shown in Figure 4A or specifically engineered features as shown in Figure 4B) can also be represented as different scattering models which can be used incorporated into other simulation frameworks such as PV Lighthouse's Module Ray Tracer or the application SunSolve. Simulations can be performed to determine the effectiveness of the light structuring not just for light at normal incidence but also at the range of incident angles that a photovoltaic module typically experiences in the field. The ability to integrate light structuring features directly into the metal forming process eliminates the need to use additional light capturing films (LCF) or ribbons (LCR) which have been successful in increasing the power generation from photovoltaic modules. Strategies which employ LCF are however typically more costly due to the increased material cost as the film must be adhered to the standard interconnection ribbon. With LCR, the structuring is introduced directly on the interconnection wire, however this process is limited to very simple structures such as ridges and high frequency features cannot easily be introduced.

Figure 5A shows a section of a polymer surface 500 with laser- structured tracks 505 which, when metallised, will extend over a solar cell 130 where they will bond with the cell metallization to extract current from the cell. Also shown is an

interconnection tab 510 which can enable interconnection between adjacent cells in the module through bonding with a second metallized encapsulant sheet during the lamination step substantially as depicted in Figure 1. The details of the surface roughening in the laser-structured tracks are not shown in Figure 5A for clarity, and different surface morphologies can be used for each of the laser-structured tracks 505 and interconnection tabs 510. For example, the use of a series of gratings (as shown in Figure 4B) for greater optical gain may be reserved for the interconnection tabs 510 for improved process throughput in the laser patterning step. It should also be noted that the interconnection tabs 510 do not need to be continuous linear regions as depicted in Figure 5A.

The laser-structured surface is then selectively made conductive by the formation of a seed layer in step 310. Preferably this is achieved by electroles sly plating a seed metal layer, although other methods can also be used. The seed metal can be Ni, Sn, Ag, Zn, W, Mo, Pd, Ti, Co, Au or alloys which include two or more of these elements . Alternatively the seed material can be a conductive polymer such as poly ( 3 , 4-ethylenedioxythiophene ) polystyrene sulfonate (PEDOT:PSS) . Figure 5B shows a section of polymer after formation of the seed layer with items 515 and 520 indicating the seed layer formed over the surface of a laser- ablated track 505 and interconnection tab 510. Since the seed material preferentially forms (e.g., plates) to roughened surfaces, it selectively deposits over the laser-ablated regions following the rough contours of the surface thereby resulting in a conformal conductive layer that coats the polymer opening. In addition to providing a light scattering surface, the laser roughness ensures strong adhesion of the seed material to the polymer surface. Strong adhesion of the seed layer material is critical for the subsequent formation of the conductive pattern (step 315) and the durability of the metallized polymer, especially if the metallized polymer is to be delivered to module manufacturers as rolls and stored in this fashion until required for use.

The seed layer is then thickened in step 315 preferably using an electroplating process. Figure 5C shows a section of polymer 500 after the formation of conductor elements 525 and 530 over the seed layers 515 and 520, respectively. It is advantageous to use Cu for the conductor elements due to its high conductivity and relatively low-cost, however other metals can also be used.

Electroplating can be performed using continuous plating equipment, where the polymer can be extended through a plating bath using R2R automation enabling continuous plating up of the elements 525 and 530 on the polymer surface 500 as the polymer is rolled from one reel to the next through the plating electrolyte.

The Cu electroplating time (and hence the final thickness of the metal tracks) can be estimated from a knowledge of the current to be collected from the cells, the width of the metal elements, the conductivity of the Cu (assumed to be the same as bulk Cu), the thickness and resistivity of the capping alloy and the maximum allowable power loss due to series resistance in the metal (preferably below 1.0%) . With smaller finger widths, the height of the plated of Cu can become very large. For example, if the power loss due to series resistance is to be reduced to less than 1% and conductor elements must collect current from the entire width of a 15.6 cm solar cell, then the plated height needs to be about 150 μπι for a 100 μπι wide conductor (shading of 4.5%) . Cu can be electroplated at current densities of 100 mA/cm² and in some cases larger rates without impacting the properties of the metal deposit providing that the chemical formulation used for the bath is tuned appropriately (see US patent 6,676,823 to Bokisa) . Furthermore, the electroplating process for laser-structured encapsulant surfaces can directly use chemical additives and pulse plating processes that have been developed and used to form Cu-plated interconnects for integrated circuits and printed circuit boards where Cu deposit thicknesses approaching 100 μπι have been used to fill vias that are 100 μπι in diameter.

A strategy to reduce series resistance losses of modules that is being increasingly employed is to use half (wafer) solar cells. This practice reduces the current that needs to be extracted from each cell and, in doing so, allows for reduced-height conductors. Cells can be fabricated on full wafers and then cleaved after cell metallization to form half or even quarter width cells which can then be interconnected in different serial and parallel configurations. This approach can reduce the amount of conductive metal that needs to be electroplated.

The process of shingling, described further below with reference to Figure 12, exploits this property with cells being cleaved into as many as 6 rectangular slices for interconnection by shingling. Initiating the polymer metallization process using laser structuring permits high aspect ratio plated conductors to be formed due to the constraining the metal to the laser-ablated groove. Additionally, the laser-structured grooves can provide additional adhesion due to the increased metal substrate surface area .

Figure 7A shows an example of a metallized thermoplastic polyolefin (TPO) sheet after Cu electroplating onto a seed Ni layer that was selectively deposited over laser-structured regions using an electroless plating process. The metallized TPO has been cut to the size of a 156 mm solar cell. The linear metal tracks adhere strongly to the TPO allowing the TPO to be rolled, folded and cut as required. The adhesion of the formed metal is not impacted by the Cu pattern formed.

Once sufficient Cu has been plated then a capping layer is formed over the Cu surface in step 320. Figure 5D shows this step for a laser-structured pattern with 535 and 540

representing the capping layers for the cell metal conductors 525 and interconnection tabs 530, respectively. Preferably the capping layer comprises a low melting-point metal alloy, such as Sn-Bi or a Sn-Bi-Ag alloy, and avoids the use of Pb . A capping Sn-Bi alloy can be plated using substantially the same

electroplating process used for step 315. This requires that the encapsulant now extends into a second bath which exposes surface 500 to an electrolyte comprising ions of the alloy components. The desired Sn and Bi fractions in the deposited metal alloy are controlled by the metal ion concentrations in the plating solution. For example, to achieve a eutectic alloy of 42 wt% Sn and 58 wt% Bi, plating can be conducted in a solution comprising 0.15 M stannous chloride, 0.05 M Bi nitrite, 0.05 M

ethylenediaminetetraacetic acid, 0.3 M citric acid and 0.2 M polyethylene glycol 400. A eutectic Sn-Bi alloy has an

electrical resistivity of 30-35 μΩ-m (20 times that of Cu) and so, although the conductivity of the capping can be increased by a reflowing process (described further below), its thickness is preferably designed for optimal bonding.

The low-melting point alloy can also be applied by screen- printing an alloy paste aligned to the plated Cu pattern. In this arrangement, the encapsulant is rinsed and dried after the Cu electroplating step 315 before passing across the stage of a screen-printer. The alignment of the screen-printed alloy paste to the Cu-plated metal pattern is achieved using an optical vision system using alignment marks adjacent to the metal pattern. Aligned screen-printing has been routinely used in solar cell metallization to achieve thickened metal fingers on the cell surface and so a process similar to that process can be used. A number of Sn-Bi alloy pastes can be used, with an example of suitable pastes being Sn43Bi57 provided by the company Metaux Blancs Ouvres .

The key advantage of applying the low melting point alloy using screen-printing is that the composition of the paste has been previously optimized and does not depend on the control of the alloy electroplating process. It also allows new alloy pastes to be readily incorporated into the metallized encapsulant manufacturing process with minimal changes in capital equipment. The disadvantage of using screen-printing for this step is that it introduces additional equipment into the manufacturing process. However, the production of screen printing equipment is well developed due to its extensive use in photovoltaic cell manufacturing world-wide and the use of screen-printing to dispense alloys has been previously demonstrated in the field of printed circuit boards . The low melting point alloy pastes can also be dispensed using standard paste dispensing equipment such as Performus™ Series Dispenser.

Before lamination the plated alloy must be reflowed to form an alloy surface that can be metallurgically-bonded (e.g., with the cell metallization) during lamination. On reflow the usually dull metal alloy colour becomes very reflective. Alloys are preferably selected such that the reflow process can be performed on the metal elements of the pattern between the temperatures of 160 and 180 °C . The reflow can be achieved by locally heating the alloy surface using focused hot air, a torch, a soldering iron, or a diode laser (wavelength in the range of 750 to 1000 nm and a power of 15 to 25 W) . For localized reflow a vision system is required to guide the heating element to ensure that it is aligned with the metal pattern on the encapsulant surface.

An alternative method for reflow is to pass the sheet of the polymeric material over a heating element or under a jet of hot air in a reel-to-reel process. The temperature of the heating element or hot air jet and timing of the flow must be tuned to achieve optimal reflow without damaging the encapsulant. A further alternative is to reflow the paste before dispensing with a heated printing head. This can be achieved by adding a heated syringe to the dispensing system such as The GPD Global® MAX II Series.

Figure 7B shows an example of a metal track 710 on a TPO sheet after Cu plating and an adjacent metal track 720 that has been capped with a Sn-Bi-Ag solder paste and reflowed after

dispensing using a heated iron tip. This local reflow can be performed without structurally impacting the polymer appearance or structure, or modifying the polymer's optical properties.

The bonding material can also comprise an electrically

conductive adhesive (ECA) which is dispensed aligned with the underlying metallized tracks. Electrically conductive adhesives can be dispensed using methods that are similar to that used for the alloy pastes. Arrangements can be used whereby alloys and ECAs are used for different elements of the metallization. For example, low-melting point alloys can be used to bond with the cell metallization and the ECAs used for cell interconnection (i.e., for interconnection tabs 128 and 148 in Figure 1) .

In an alternative laser structuring process shown in Figure 4C, an increased laser fluence is used to ablate openings through the entire polymer layer rather than trenches as shown in

Figures 4A and 4B. This approach is particularly advantageous when the metal pattern is required to conduct large currents between cells because, if the openings are filled with metal in a subsequent electroplating process, very high aspect ratio metal conducting elements can be achieved. Polymer encapsulants used in photovoltaic modules typically have thicknesses in the order of 200-550 μπι and so the metal thickness can approach these values.

With this arrangement, preferably the openings are formed using a C0₂ laser. Optimization of the laser power can result in the formation of linear openings through the entire thickness of the polymer in a single pass with minimal structural damage or optical modification of the adjacent polymer. Openings can also be made in the polymer using other methods (e.g., using a punch, stencil or a mechanical cutter) .

Unlike the process 300 in which a laser-ablated trench is formed, the alternative process 330 which is summarized in Figure 3B, involves the formation of openings throughout the entire thickness of a polymer sheet and the conductors are formed using an electrode placed on a surface of the polymer. Then, as shown in Figure 4C, structuring or texturing of the plated metal in the opening can be achieved by using a

structured or roughened electrode surface such as 450 in Figure 4C.

This alternative process proceeds by first forming the openings in the polymer in step 335. Preferably these openings are formed by a laser, however as mentioned above other opening methods can also be used. As shown in Figure 6A, preferably the polymer 605 is coated on one surface with a sacrificial protective layer or a thin layer of adhesive material 610. Encapsulants commonly used for Si photovoltaic modules are frequently packaged with an interlayer to protect individual surfaces from undesirable tacking of polymer surfaces, and these interlayers can be used as the sacrificial protective layer 610. Alternatively, an adhesive material can be tacked to the polymer surface in a roller-based process to form the sacrificial protective layer 610 before laser structuring. The adhesion of the sacrificial protective layer to the polymer surface can be improved by the application of heat to soften either the polymer and/or the material of the sacrificial protective layer. Alternatively, the sacrificial protective layer can be formed by spray coating a thin layer of polymer which can be annealed at low temperature to form a film.

In the laser structuring step, the sacrificial protective layer 610 is ablated along with the polymer 605 to form a series of openings 615 and 620 that extend through the polymer 605 as shown in Figure 6B.

Step 335 can be performed with the coated polymer being placed on a stage in a cutting/ablation tool. Once the openings are formed, the patterned polymer is then tacked to the

electroplating electrode, or mandrel 450 in step 340. The tacking can be achieved by warming the polymer to a temperature that allows the polymer to soften without deforming or melting. For example, for EVA, tacking can be achieved by heating the mandrel and polymer to a temperature of 65 to 85 °C, and more preferably 80 °C . Alternatively, the polymer 605, with the sacrificial protective layer 610, can be tacked to the mandrel 450 and then the openings can be formed. The latter process is preferable when high resolution alignment is required as it eliminates risks of polymer deformation during the process of tacking the polymer to the mandrel.

The mandrel 450 can be structured as shown in Figure AC. This allows for the conductive elements that are electroformed through the openings to be structured/patterned for optimal light trapping (see Figure 4D) . The mandrel pattern can comprise a two dimensional (e.g., series of ridges and valleys) or three dimensional (e.g., random or periodic pyramids) pattern. The structuring of the mandrel 450 can be performed using a moulding process, a laser or by chemically etching the mandrel material. This mandrel structuring process allows for more pattern flexibility than possible by patterning/structuring Cu wires or interconnection tabs, since the latter structuring approach needs to be performed on the wires as they are fed from R2R before being dip-coated in molten solder. Preferably the mandrel is re-used for many processes and the structuring on the surface retains its geometric properties for many plating processes.

After tacking the polymer to the mandrel and forming the pattern of openings (steps 335 and 340) , a thin seed barrier layer 625 is preferably coated over the polymer surface covering the sacrificial protective layer 610 in step 345 as shown in Figure 6C. This barrier material preferably comprises a refractory metal binary or ternary alloy comprising elements such as Ni, Ti, Ta, W, Co, Zr, N and Si (e.g., TiN TaN, and TiZrN) . It acts as a barrier for Cu diffusion into the polymer and prevents any reactions between the Cu and the polymer. For example,

degradation of the polymer can result in acidic or alkaline byproducts that can corrode the Cu conductors formed in the polymer. Materials and processes, substantially as described for the formation of barrier materials for Cu in integrated circuit fabrication can be used. For example, the barrier material can be deposited using sputtering, evaporation or electroless deposition. High throughput deposition of the barrier material can be achieved using R2R processing equipment.

The barrier material 625 covers the entire surface of the sacrificial protective layer 610 and also extends into the openings 615 and 620 where it uniformly coats the exposed polymer surfaces. The barrier material 625 also coats the surface of the mandrel 450 exposed in the openings in the polymer. The mandrel 450 is preferably designed such that it has weak adhesion to the barrier material 625. Then, as depicted in Figure 6D, the sacrificial layer 610 is removed taking with it the coating barrier material 625 and leaving behind only the barrier material that coats the surface of the exposed polymer in the openings. The surface layers in the openings, 625 and 630 in Figure 6D (see also 435 and 438 in Figure 4C), present a barrier for Cu diffusion into the polymer in the fabricated photovoltaic module.

In an alternative arrangement, the sacrificial protective layer 610 is not used, and instead the barrier material 625 is applied directly to the polymer 605 surface after forming the openings 615 and 620. The barrier material can then be removed,

preferably before step 350 using a chemical polishing method, once again substantially as performed for the formation of plated Cu interconnects for integrated circuits. During this polishing step the surface properties of the polymer 605 can be modified to increase the wetting of the solar cell surfaces during module lamination. This polishing step can also be performed after the Cu conductors have been electroformed, however this process is less preferable because of the larger amounts of material waste that results .

Then in step 350, the openings are filling by electroplating in a process where current is applied to the mandrel 450 whilst the polymer openings 615 and 620 are exposed to the electrolyte. Barrier materials such as TiN and TaN provide strong adhesion to electroplated Cu whilst also ensuring that Cu does not penetrate into the polymer during the operating lifetime of the resultant photovoltaic module. The adhesion of the electroplated Cu can be further increased by depositing a seed layer of Cu over the barrier material immediately after the barrier material is deposited to eliminate the possibility of oxide formation. The electroplating can be performed substantially as described for step 315 in Figure 3A and can use either direct current or pulse plating processes. Pulse plating is advantageous for very narrow openings to ensure that the openings do not become depleted of metal ions. Figure 6E shows a section of polymer 605 with Cu conductors 635 and 640 filling the openings 615 and 620 which are lined with barrier material 625 and 630. After completion of the electroplating process, the metallized polymer is removed from the mandrel in step 355. This results in a metallized polymer such as depicted in Figure 4D. The Cu conductor 430 extends through the entire thickness of the polymer and is totally enclosed by the barrier material 435 and 438. The pattern of the structured mandrel 450 is imprinted in both the barrier material 438 and Cu of the conductor 430.

Steps 350 and 355 can be performed in a R2R process with the mandrel 450 extending under the polymer in the plating bath. Once plating is complete, the polymer can be removed in step 355 from the mandrel 450 to leave the polymer complete with a conductive metal pattern that extends all through its entire thickness .

Figure 8 shows an electroplating arrangement with a structured mandrel (cathode) 810 and anode 820 extending across the plating bath filled with an electrolyte 830. The anode and cathode are connected to a power source 825 external to the bath. The polymer being metallized 840 is held to the structured mandrel 810 by a series of rollers (e.g., 860) as it moves through the plating bath supported by the mandrel 810. The upper surface of the polymer 840 is exposed to the plating electrolyte 830 and metal ions can extend into the openings where they can be reduced at the cathodic metal surface. On completion of the electroplating process the polymer is rinsed (not shown in Figure 8) and rolled onto the collecting roll.

In step 360, the metallized polymer is then capped with a bonding material as described for process 300 in Figure 3A.

Figure 4D depicts a filled opening 430 with a solder capping 440 and a structured metal surface on the opposite surface. The surface tension introduced during the reflow step acts to draw the plated Cu upward in the openings so that the metal in the openings on the non-capped surface is not flush with the surface and consequently does not come directly in contact with either the front or back sheet of the module during lamination.

As described with reference to Figure 3A the bonding material can comprise an electroplated alloy that is electrodeposited whilst the polymer remains tacked to the structured mandrel 450. This alternative process has the benefit of eliminating the requirement for alignment in step 360.

The process 330 can also be achieved by using two sheets of polymeric material. The first sheet of the polymeric material, which is metallized, is engineered to be the required metal thickness. The second sheet, which is not metallized, provides the remaining encapsulation. This variation can ensure that there are no conductive paths in contact with the module' s front or back sheets that can provide detrimental current pathways in the presence of high voltages that can lead to poor resistance to potential-induced degradation.

The metallized polymer can also be fabricated by printing a conductive seed layer pattern by inkjet printing, aerosol printing or screen printing and then thickening this seed layer in a subsequent electroplating step. Conductive seed layers can be formed by printing metal nanoparticle inks that have been subsequently heated or light-treated to eliminate solvents and sinter the particles, metal-organic decomposition (MOD) inks or conductive polymers such as poly ( 3 , 4-ethylenedioxythiophene ) - poly ( styrenesulfonate) (PEDOT:PSS) .

In the case of metal nanoparticles , annealing can be achieved by using either a laser or a photoflash to eliminate any organic functional groups used to prevent aggregation of the inks. In another variation, the seed layer can be printed using a reactive inkj et/aerosol printing process in which a Cu salt (e.g., Cu formate) is first printed and then annealed at a low temperature of about 140°C to self-reduce the Cu ions to metallic Cu . The latter process presents some advantages over the printing of nanoparticle inks. First, the inkjet printing process is more reliable due to no ink particle aggregation and nozzle clogging, consequently a higher metal loading can be used in the ink. Second, the deposited Cu does not oxidise due to the reducing environment generated through the self-reduction reaction which also ensures a reduced polymer surface for increased adhesion.

Screen-printing can also be used to form the seed layer on the polymer surface. A range of Ag inks that are substantially similar to those used for cell metallization can be printed on the polymer and then annealed at temperatures of ~ 120°C to remove solvents. The seed layer needs only to be 1-2 μπι thick to ensure sufficient conductivity and so should not require excessive amounts of Ag. Alternatively, more recently developed Cu pastes can also be used for this step.

This process 370 is shown as a flowchart in Figure 3C. After the seed layer is deposited and heat-treated in step 375, it is then electroplated to thicken the pattern to the required

conductivity in step 380. The metal pattern is then capped with a bonding material as described for process 300.

The seed layer can also be formed directly on the polymer by a vacuum deposition method (e.g., sputtering) using a shadow mask. This process is conceptually similar to process 370 summarized in Figure 3C as the seed metal pattern is formed on the surface of the polymer. The advantage of this process over the printed seed layer is that no additional chemistry is required to form the Cu or remove functional groups which can potentially remain as a contaminant in the plated metal pattern. It can also permit a broad range of different seed layer metals (e.g., Ti, Ni, Ag or metal alloys that may enhance barrier layer properties) . The seed layers are formed in one step at room temperature within several minutes with good uniformity and high adhesion to the encapsulant surface. The sputtering step can be performed using R2R sputtering equipment and is compatible with R2R

electroplating systems.

With the printing and sputtering seed layer alternatives, if light trapping functionality is required then the deposited seed layers can be aligned to a laser-structured pattern. This is readily achievable with inkjet and aerosol printing, as the printers are routinely equipped with vision systems for alignment; however it can be more challenging when sputtering through a shadow mask, especially if R2R sputtering is employed for a high throughput process.

This alternative fabrication method for the metallized

encapsulant where a seed layer is formed through either a printing or sputtering process is shown using Figures 9A, 9B and 9C. The seed layer is first deposited on the encapsulant 900 in a pattern comprising of linear cell-level conductors 905 and interconnection tabs 910. The seed layer needs to be

sufficiently conductive that the electroplating step results in a uniform thickness of plated Cu over the entire pattern. Figure 9B shows the polymer surface after the electroplating step with 915 and 920 indicating the electroplated Cu in contact with the seed layer 905 and 910, respectively. Electroplating results in conformal metal growth and so the width of the pattern elements is now wider than that of the corresponding seed layer elements. The thickened metal tracks are then capped (see Figure 9C) . The bonding material (e.g., low melting temperature alloy) 925 and 930 extends over conductive elements 915 and 920, respectively.

On completion of the abovementioned steps, the metallized polymer can be either cut into module sized sheets or provided to the photovoltaic module producer in roll form which can then be cut into the appropriate size sheet when required. The metallized polymer can also be provided to manufacturers in custom sizes according to demand, as changes in the

metallization pattern can be readily achieved with minimal change in the metallized polymer manufacturing process.

The process of fabricating a photovoltaic module using

metallized sheets of polymeric material is described now with respect to Figure 10. The process 1000 commences by laying out the module back sheet in step 1010. The back sheet can be a lightweight opaque composite material for monofacial modules.

Alternatively, glass or a transparent polymeric material can be used for bifacial modules. Preferably a 'solar glass', which has antireflective and light scattering capability integrated, is used. A first metallized encapsulating sheet of polymeric material is then laid over the back sheet in step 1020. It can be extended over the back sheet from a roll and then, once aligned over the back sheet, cut to size or simply laid as a pre-cut sheet. The metallized polymer surface is aligned using alignment marks provided on the layout assembly surface using an optical alignment system with the surface exposing the

conductive metal pattern (with its bonding surface) facing upwards in readiness for bonding with the solar cells of the module .

Then in step 1030 pick and place automation is used to place solar cells on the first encapsulant sheet according to the layout pattern. Preferably, the solar cells have conductive regions, such as metal fingers (such as shown in Figure 1), that will bond with conductive metal elements in the metallized polymer during lamination. Alternatively, the solar cells can be coated with a conducting oxide which is preferably substantially transparent and forms a low-resistance electrical contact with the conductive metal elements in the metallized polymer during lamination. Preferably the layout placement accuracy is 10 μπι ± 3μπι, and more preferably 10 μπι ± 1 μπι, however the placement accuracy can be sacrificed for faster placement if higher processing throughput is required.

Once all the cells are laid out then the second encapsulating sheet of polymeric material is laid over the cells in step 1040, followed by the front sheet in step 1050. If electrical contact is required to both surfaces of the solar cells, then the second sheet of polymeric material is also metallized as discussed for the first sheet. The front sheet can comprise glass or a transparent polymeric material that is substantially

transparent. The assembly is then moved into the laminator where it is laminated in step 1060.

The lamination process can be customized for the type of encapsulating polymer used and the solder and bonding processes employed. For example, plated and printed metal alloys can require different bonding conditions and so the lamination process must be tuned to these requirements. Additionally, if ECAs are being used for the between cell bonding, then the lamination process may also need to be adapted for these requirements. After lamination the module is completed by adding the frame (if required) and the module junction box (step 1070) .

Figure 11 is a cross-sectional schematic of a section of a thus- fabricated bifacial module (before lamination) 1100. It shows an upper and lower metallized encapsulant sheet of polymeric material 1110 and 1120, and between the sheets lie two adjacent solar cells in the module. Each cell has an array of conductive fingers 1130 formed on both cell surfaces. The cells and sheets of polymeric material are placed in alignment such that the interconnection tabs 1140 are aligned and the cell-level conductors 1150 formed on the metallized sheets of polymeric material extend perpendicularly across the solar cells in contact with the solar cell fingers. In the figure, the spacing between the solar cells has been exaggerated in order to make evident the way adjacent cells are interconnected via the conductive metal patterns formed in the polymer sheets 1110 and 1120.

The spacing between the individual solar cells in the

photovoltaic module can be optimized for optical performance and reliability. Because the interconnection tabs are structured (as described previously, for example with reference to Figures 3 and 4), light incident on them is scattered thereby increasing the probability of total internal reflection at the front sheet or module glass. Consequently, some level of spacing between solar cells does not negatively impact optical performance and, if light scattering is optimized, then can actually enhance module efficiency. For example, a cell-to-cell spacing between 3 to 5 mm can be advantageous in terms of the optical performance of a photovoltaic module.

Although the module fabrication process has been described for bifacial cells where an electrode exists on both surfaces of the solar cell, it can also be used to interconnect solar cells where both polarities of contact are formed on the rear surface. It should be clear from the above description that a metal pattern comprising a linear array of metal conductors can achieve this interconnection by element of the conductive metal pattern in the polymer extending between two adjacent cells and connected n-type and p-type regions of the adjacent solar cells. This variation highlights the potential advantage of providing most of the metal required to conduct current between cells in a module on the encapsulant surface or within the encapsulating polymer rather than on the individual solar cells. Reducing the amount of cell metallization can directly reduce metallization costs by reducing the amount of Ag consumed. Furthermore, if the solar cells are Cu-plated, then the use of thin metal fingers can provide advantages in terms of increased adhesion compared to cells with very thick Cu-plated fingers. Reducing the mass of metal required to be deposited or printed on the solar cell surfaces, also makes possible the use of thinner Si wafers, which can further reduce costs of the photovoltaic modules.

Although the example provided above uses a metal pattern comprising linear elements, the metal pattern is not limited to linear elements and can be customized for different

applications. It also allows the flexibility of adding other circuit elements such as printed bypass diodes, conductive elements allowing different series and parallel arrangements of cells (e.g., for specific current-voltage applications) and integration with string level DC/DC power optimizers. The latter is particularly advantageous as the use of field-replaceable string-level electronics eliminates the need for bypass diodes to be encapsulated into the module and can allow for longer module lifetimes and thereby reduced levelized costs of electricity. Integration with additional module circuit elements is challenging with existing traditional soldered

interconnection and the wire-based interconnection methods such as SWCT and MultiBB.

The described encapsulating polymer metallization process can also be used to support shingled modules which are designed to eliminate the need for soldered interconnection by stacking cells directly on the edges of adjacent cells with ECA providing the bonding between cells. Although conceptually very simple, this shingled module architecture, which was first described by Dickson in US 2,938,938 granted in 1960, has been difficult to achieve in a manufacturing environment due to the complexity of the process and reliability issues that arise due to the stress introduced by the stacking of cells in the module. One way in which this stress can be reduced is to align solar cells in shingled sub-assemblies with tabs connecting the shingled sub- arrays in parallel. This configuration is described in US patent application 2016/0163902 (Podlowski) assigned to Pi Solar Technology Gmbh . The polymer metallization process described in this application could be used to interconnect shingled subassemblies as shown in Figure 12. Item 1200 depicts a subsection of a shingled module which comprises shingled assemblies 1210 in connection with conducting elements 1220 provided by a first (rear) metallized encapsulating sheet of polymeric material and 1230 provided by a second (front) metallized encapsulating sheet of polymeric material. Unlike the rigid ribbons suggested in US patent application 2016/0163902, the softer Cu tracks of appropriately engineered thickness can reduce stresses induced by the lamination of the shingled sub- assemblies. The use of the parallel connection of the series connected shingled modules also results in modules that are less susceptible to shading losses without the use of bypass diodes. As can be appreciated, this represents an example shingled arrangement and shingled cell arrays and connecting elements can be designed without the necessary requirement of linear segments which is typically required for all methods involving wires and tabs .

A key advantage to this new interconnection method is that the encapsulant metallization process can be performed by an entity other than the photovoltaic manufacturer, thereby reducing the complexity of the module manufacturing process.

The metallized encapsulating polymer can also be used for other applications, being especially useful for devices requiring interconnection and encapsulation. It can be used to embed or encapsulate other semiconductor devices (e.g., light-emitting diodes), passive and active circuit elements, sensors and antennae. Incorporation of antenna elements can enable the production of flexible RFID transponders, that can be adhered to a range of flat, curved or textured surfaces with the polymer providing both and adhesive and encapsulating functionality.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this

application .

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps .

Claims

The Claims:

1. A method of forming a conductive metal pattern on at least a portion of a polymeric material, the method comprising the steps of:

providing the polymeric material;

forming a predetermined pattern on or within the polymeric material ;

forming a conductive metal pattern aligned with the predetermined pattern by depositing a first material using an electrochemical process; and

applying a second material to at least a portion of the formed conductive metal pattern, the second material being suitable for bonding to at least the portion of the formed conductive metal pattern and to one or more conductive surfaces of an electrical device.

2. The method of claim 1 comprising:

providing at least one electrical device having one or more conductive surfaces ;

contacting the one or more conductive surfaces of the at least one electrical device with the second material applied to the at least a portion of the formed conductive metal pattern; and

annealing the polymeric material with the one or more conductive surfaces of the at least one electrical device such that the applied second material bonds to the one or more conductive surfaces of the at least one electrical device and the at least a portion of the formed conductive metal pattern.

3. The method of claim 2, wherein the annealing is conducted at a temperature up to 160°C.

4. The method of claim 2 or 3 wherein the annealing is conducted in an environment in which an ambient pressure is 40- 60kPa.

5. The method of any one of the preceding claims, wherein the electrical devices comprises one or more of: a semiconducting device such as a solar cell and light-emitting diode, an electrical circuit containing passive and active elements, and a sensor or an antenna element.

6. The method of any one of the preceding claims further comprising lining at least a portion of the polymeric material at the formed predetermined pattern with a third material prior to forming the conductive metal pattern aligned with the predetermined pattern.

7. The method of claim 5, wherein the third material is a refractory metal or metal alloy comprising one or more elements from the group of Ti, Ta, W, Mo, Ni, Pt, N, Co and Si.

8. The method of any one of the preceding claims wherein forming a predetermined pattern on or within the polymeric material comprises:

applying a masking layer to a surface of the polymeric material; and

forming the predetermined pattern within the masking layer and polymeric material.

9. The method of claim 6 or 7 or claim 8 when dependent on claim 4 wherein lining at least a portion of the polymeric material at the predetermined pattern comprises:

depositing the third material over the predetermined pattern that has been formed in the masking layer and polymer;

and

removing the masking layer such that the third material only covers the polymeric material at the predetermined pattern .

10. The method of claim 8 or 9 wherein the masking layer is attached to the polymeric material by application of heat and/or pressure . - se ¬

11. The method of any one of the preceding claims wherein the predetermined pattern includes at least one recess or hole that penetrates through the entire thickness of the polymer and wherein the metal of the conductive metal pattern penetrates through or into the at least one recess and/or into the at least one hole.

12. The method of claim 6 or any one of claims 7 to 11 when dependent on claim 6 wherein the third material is arranged to provide a diffusion barrier for the first material preventing or reducing the likelihood of diffusion of the first material into the polymeric material.

13. The method of claim 6 or any one of claims 6 to 10 when dependent on claim 6 wherein the third material is arranged to provide a seed layer facilitating the adhesion of the first material when it is deposited on the surface of the third material .

14. The method of any one of the preceding claims wherein the second material comprises a low-melting point metal alloy.

15. The method of any one of the preceding claims wherein at least the portion of the conductive metal pattern and the applied second material are heated before the annealing to reflow the applied second material to form a uniform alloy coating over at least a portion of the conductive metal pattern.

16. The method of claim 14 or 15 when dependent on claim 14, comprising applying localized heat to form a uniform metal alloy coating over at least a portion of the conductive metal pattern.

17. The method of claim 15 or claim 16 when dependent on claim 15, wherein reflowing is performed at a temperature between 160°C and 180°C.

18. The method of claim 15 or claim 16 when dependent on claim 15, wherein reflowing is performed at a temperature between 140°C and 160°C.

19. The method of any one of the preceding claims, wherein the second material comprises an electrically conductive adhesive.

20. The method of any one of claims 1 to 5, wherein the step of forming the predetermined pattern aligned with the surface of the polymeric material comprises:

structuring one or more regions of the surface of the polymeric material; and

selectively depositing the third material only on the areas of the surface of the polymeric material that are structured .

21. The method of claim 20, wherein the structuring is performed using a pulsed laser having a pulse duration of less than 15 ps .

22. The method of claim 20 or claim 21, wherein the

structuring is performed using a UV laser.

23. The method of any one of claim 20 to 22, wherein the third material is deposited using an electroless deposition process and can be one of the following materials: Ni, Sn, Ag, W, Mo, Co or alloys of these elements.

24. The method of any one of claims 20 to 23, wherein the structuring results in an opening which extends through the thickness of the polymeric material and the conductive metal pattern is formed by electroplating and extends throughout the thickness of the polymeric material.

25. The method of claim 6 or any one of claims 7 to 24 when dependent on claim 4 wherein lining a surface of the polymeric material at the formed predetermined pattern with a third material comprises sputtering a metal on the surface of the polymeric material or masking layer to form a conductive seed layer .

26. The method of any one of the preceding claims, wherein the predetermined pattern on or within the polymeric material is formed using one of, or a combination of, inkjet printing, aerosol printing or screen printing to deposit a conductive seed layer .

27. The method of any one of the preceding claims, wherein at least a portion of the formed conductive metal pattern is formed to increase scattering of light incident on the conductive pattern surface.

28. The method of claims 27, wherein the light scattering of the conductive metal pattern is achieved by using an electroplating mandrel which has a structured surface.

29. The method of any one of the preceding claims, wherein the step of forming a conductive metal pattern includes an

electroplating step.

30. The method of claim 29, wherein the electroplating step is performed using a reel-to-reel process in which the polymeric material passes through an electrolyte comprising at least one metal ion salt.

31. The method of any one of the preceding claims, wherein the formed conductive metal pattern contains Cu .

32. The method of any one of the preceding claims, wherein the second material is applied using either a printing or dispensing process .

33. The method of any one of claims 1 to 31, wherein the second material is formed using an electroplating process which involves a chemical bath comprising metal ions corresponding to the elements of the alloy.

34. A method of fabricating a photovoltaic module comprising at least two solar cells, with at least two of the solar cells having one or more conductive regions on at least one surface, the method comprising the steps of:

providing a module back sheet, the back sheet comprising either a polymer, polymer composite or glass material;

providing a first sheet of a polymeric material, the polymeric material having a predetermined conductive metal pattern with at least a portion coated with a bonding surface comprising of a second material exposed on a surface of the polymeric material;

arranging the first sheet of the polymeric material over the module back sheet;

arranging the plurality of solar cells over the first sheet of the polymeric material, each solar cell having at least one conductive region on a surface of the solar cell, and the solar cells being arranged such that the at least one conductive region from each solar cell is in contact with the said bonding surface exposed on the first sheet of the polymeric material; providing a second sheet of a polymeric material arranged to cover the plurality of solar cells;

arranging a substantially transparent front sheet over the second sheet of the polymeric material; and

laminating the module back sheet, the first sheet of the polymeric material, the plurality of solar cells, the second sheet of the polymeric material and the front sheet;

wherein, during lamination, at least one conductive region of the plurality of solar cells bonds with a portion of the bonding surface exposed on the first sheet of the polymeric material and an electrical circuit connection is formed between at least one pair of adjacent solar cells.

35. The method of claim 34, wherein the first sheet of the polymeric material is fabricated in accordance with the method of any one of claims 1 to 33.

36. The method of claim 34 or 35 wherein the electrical connection between two adjacent solar cells in the photovoltaic module is formed by an element of the conductive metal pattern in the polymeric material that extends between the two adjacent solar cells .

37. The method of any one of claims 34 to 36, wherein the electrical connection between adjacent solar cells in the photovoltaic module is formed by bonding between aligned bonding surfaces exposed on both the first and second sheets of polymeric material, said bonding occurring during the laminated step .

38. The method of any one of claims 34 to 37, wherein the conductive regions on the solar cells comprise metal elements

39. The method of any one of claims 34 to 38 wherein the conductive regions of the solar cells comprise conducting oxide regions .

40. A photovoltaic module manufactured in accordance with the method of claim 34 to 39.