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Photovoltaic Cells Evolve with Pressure-Sensitive Adhesives
7/22/2010
As photovoltaic (PV) cell technology rapidly evolves from amorphous and crystalline silicon forms to thin film technologies, PV manufacturers are confronted with a new array of bonding and manufacturing process challenges.
Click image to enlargeAs photovoltaic (PV) cell technology rapidly evolves from amorphous and crystalline silicon forms to thin film technologies, PV manufacturers are confronted with a new array of bonding and manufacturing process challenges. While liquid epoxies, ethylene-vinyl acetate (EVA) hot-melt adhesives, soldering, and mechanical fasteners have long been accepted bonding methods in the assembly of photovoltaic (PV) cells, pressure-sensitive adhesives (PSAs) are emerging as a viable option.
PV product designers are facing many of the same issues electronics manufacturers faced when electronic devices evolved to faster, thinner profiles built upon highly complex and sensitive components in a smaller product footprint. Formulators of PSAs capitalized on the opportunity to provide electronically clean, reliable bonding options that featured a controlled adhesive thickness. Today, PSAs are highly sophisticated polymer formulations that deliver reliable bonds with added functionality that enhances the capabilities of a customer’s end product, while streamlining their manufacturing processes.
Benefits of PSAs
PSAs have been used for decades as functional components in the electronics, aerospace, automotive, medical, packaging, food, appliance, and construction industries. They offer a controlled thickness layer of adhesive that is immediately tacky at room temperature and only requires appropriate lamination processes for application during assembly. PSA tapes are clean to apply, easy to handle, reduce exposure to hazardous chemicals, and do not require cold storage.
When manufacturing speed is emphasized, film-based PSAs are often integrated into assembly processes to replace liquid epoxy adhesives because they offer a very clean and flexible delivery format. PSAs are available in a number of constructions, including transfer adhesives and single- or double-sided formats produced in large continuous rolls – precisely slit or cut to accommodate the needs of the customer’s manufacturing process or application. Slit rolls can be further converted a number of ways, including cut into precise die-cut components.
In addition to the many process benefits PSAs offer, their flexible and conformable format make them an attractive bonding option to manufacturers of thin film PV modules because they eliminate local stresses common to other bonding methods, thereby increasing a cell’s product life span. The viscoelastic properties of PSAs also provide an inherent advantage in environments that experience a wide range of temperature extremes.
Clean and Inert
Outgassing refers to the unwanted chemicals that can be emitted from an adhesive or other component while the finished device is in use. These emissions can contribute to contamination that may cause fogging of electronic components, corrosion of conductive surfaces, or component delamination, which can create electrical interconnection issues.
The growing complexity of sensitive electronic components over the last decade has significantly shaped the way adhesive manufacturers formulate electronically clean adhesives to demonstrate low outgassing characteristics. Producing PSAs in a controlled manufacturing environment is one way adhesive manufacturers assure a quality product. However, the major difference is in the careful consideration that is paid to the selection of raw materials, substrates, and release liner design. This attention to detail results in PSA tapes that are both acrylic acid- and organotin-free, demonstrate low extractable ions, and provide resistance to corrosion and environmental aging. In addition to considering outgassing characteristics, PV product designers should select an adhesive that remains chemically inert and offers long-term stability when exposed to high temperatures and direct sunlight.
Adhesive Research's PSA delivers reliable conductivity in solar applications.One of the most versatile adhesive technologies used in the electronics industry that can benefit PV manufacturers is electrically conductive PSAs for electrical interconnections between cells. While soldering is commonly used for interconnections in crystalline and amorphous PV cells, it is not the preferred approach for electrical interconnections in thin film modules because it creates local hot spots that create stresses; and it is not the most efficient process for high-volume manufacturing. An electrically conductive PSA should be considered a viable alternative to soldering, offering a thin, flexible bond line in a continuous roll format to simplify the manufacturing process.
One application example of electrically conductive adhesives being applied today is the use of a tin-plated copper foil, which is coated with an electrically conductive PSA as an alternative bus bar material for delivering highly-reliable conductivity in thin film PV modules. Adhesives Research makes such a PSA with its homogenous carbon-based electrically conductive adhesive technology. The electrical current generated in the module’s semiconductor is extracted by contacts on the front and rear of the cell. Widely spaced thin metal strips, or fingers, transport the electrical current generated within the cells to a larger metal bus bar strip to efficiently gather the electrical current from the individual fingers and transport the current from the solar module to a nearby junction box.
Adhesives Research’s PSA, offers reliable conductivity because the conductive particles, within the adhesive, form strong carbon chains for good point-to-point conductivity. The chains are flexible to provide movement with the adhesive as it expands or contracts against bonded substrates during temperature changes. This flexibility results in uninterrupted electrical contact for reliable electrical interconnections.
Electrically conductive PSAs can be formulated for specific conductivity and performance attributes including X, Y, and Z axis conductivity. Performance characteristics can be customized based on the type of carrier used, such as metal foil or metalized fabric substrates, as well as the types of fillers, or combination of fillers, used in the adhesive to deliver the desired results. When selecting an electrically conductive adhesive, PV product designers should select an adhesive that is capable of delivering multiple conductive pathways to assure stable and reliable conductive performance while resisting corrosion during thermal cycling and exposure to an array of environmental stresses.
Optically Clear
With the advent of thin film technologies, some of which are flexible constructions, PV manufacturers should consider alternative choices to EVA-based adhesives that enhance their product constructions. By integrating a high-performance optically clear PSA, a product designer can preserve the flexibility of the product’s overall construction while taking advantage of the adhesive’s optical properties to bond multiple layers in a thin, flexible profile. Some hybrid cells combine photovoltaic cells with a solar concentrator to produce more current, even in low light conditions. Optically clear PSAs can be used in the construction of the concentrators to bond acrylic or polycarbonate sheets to reflector sheets.
Optically clear PSAs are currently employed to bond films and glass layers in the assembly of graphics, display, and touch screen applications – where reliable optical bonds across an entire plane or surface are desired. As the needs for PV modules become more complex, manufacturers can take advantage of the functional benefits optically clear adhesives can bring, including a reliable, defect-free bonding surface that demonstrates low haze, high clarity, and light. This technology also demonstrates long-term durability when exposed to a variety of external environmental factors such UV light exposure, humidity, and temperature extremes.
Some PV components may be sensitive to outgassing, so product designers should consider optically clear PSAs that also demonstrate low outgassing of volatile organic compounds (VOCs) while retaining optical transparency and clarity. Acrylic and silicone formulations are typically used in optically clear PSAs. The final adhesive formulation is determined by factors required for the application such as material compatibility and desired refractive index, as well as any temperature and barrier performance requirements.
Typically PV manufacturers use EVA-based adhesives in combination with butyl caulk rubber edge sealants, or liquid epoxy adhesives, for encapsulation to prevent moisture ingress into the module. These technologies have a long history in PV cell production and have gained widespread acceptance; however, they do present some shortcomings. It is well known that EVA is prone to break down slightly with prolonged UV exposure, resulting in acetic acid contamination, which can be detrimental to the cell. Also, liquid epoxies are messy to apply and create mechanical stresses as a result of shrinkage from curing.
Moisture permeability of adhesives in both the transverse and Z-direction can be detrimental to the life of a photovoltaic cell. Therefore, any bonding system selected to replace the current encapsulation technology must have superior moisture barrier properties in all directions combined with sufficient bond strength. When selecting a PSA for encapsulation, PV manufacturers should take care to select a product that has sufficient rheological and moisture-resistant properties, as well as the appropriate thickness to sufficiently perform as a true encapsulate. PSA systems have been developed to meet these requirements for thin film PV applications (25µ to 50µ), and adhesive manufacturers continue their development of technologies to address the needs of thicker profile cells.
Today PSAs can be integrated into the process for encapsulation in thin film applications, assuring manufacturers a consistent adhesive thickness with dimensional stability for well-defined bond lines. In addition to ease-of-use, PSAs offer several other manufacturing efficiencies to this application including no post-heat program to complete cure or control shrinkage, as well as the ability to pre-cut parts to shape for a perimeter seal. The adhesive bond line remains flexible, eliminating cracks when bent.
As the interest for alternative energy resources continues to grow, manufacturers of PV cells are seeking ways to not only enhance their technology, but to also automate their assembly processes to deliver reliable and competitively priced modules. To that end, manufacturers must turn to products that deliver on the promise of creating reliable bonds with enhanced functionality. Having proven experience in enhancing product functionality in the electronics and other industry, PSA manufacturers are responding to the solar industry by developing enabling technologies for superior bonding combined with other functional capabilities to enhance PV product designs.
Adhesive Research Inc.
Glen Rock, PA
adhesivesresearch.com