Engineering polymers are playing a significant role in the development of lightweight front sheets for photovoltaic (PV) modules in the solar power industry. Although these high-performance materials have been used for many years in back sheets and in encapsulant layers in PV applications, they are now also being employed as lightweight alternatives to glass as front sheets for modules. The weight reduction afforded by this substitution allows the solar power industry to access new market segments, and consider innovative paths for component manufacturing.

A few years ago, the production of a c-SI PV module under the IEC 61646 standard was considered suitable and satisfactory for targeting all market segments, including residential, industrial, commercial roofing, and ground-mounted fields. Since the PV market has become more mature, however, more targeted designs and quality improvements are increasingly being sought. Design improvements are meeting specific needs and constraints, such as a high moisture barrier in tropical zones, a weight reduction in roof-mounted systems, and abrasion and fouling resistance in desert areas.

At the same time, developments in crystalline silicon inorganic thin-films (cadmium telluride, copper indium selenide, and amorphous silicon), organic thin-films (organic PV), and dye-sensitized solar cells have changed material requirements for engineering polymers used in PVs, including the need for low oxygen and moisture exposure.

Meeting performance demands
To address the escalating expectations for differentiation, and to satisfy the weight reduction some PV market segments now require, UV-stable fluorinated polymers—such as ethylene tetrafluoroethylene (ETFE) and ethylene chlorotrifluoroethylene (ECTFE)—are increasingly being considered as a glass replacement for front sheets. An efficient front sheet must provide high light transmission (more than 90% in the relevant range of the solar spectrum), and deliver outstanding weather resistance (lasting over a lifetime of 25 years under direct exposure to sunlight and other elements, such as rain and hail), while guaranteeing adhesion to ethylene vinyl acetate (EVA).

The replacement of glass with engineering polymers in PVs allows more ultraviolet light in the 200- to 390-nanometre (nm) wavelengths to reach encapsulants made of EVA, which can cause yellowing and degradation. A challenge for polymeric front sheets has been the development of materials that can shield encapsulant materials from UV light, while still allowing a high level of total energy transmission.

To date, solutions have included:

• Organic UV stabilizers, which degrade over time;
• Stable inorganic materials; and
• Materials that shift UV light to longer wavelengths.

Front sheets made of polymeric film must also allow an acceptable level of water or oxygen ingress to sensitive inorganic thin-films. Thanks to significant research, today’s solutions based on fluorinated polymers can meet this requirement, with water vapor transmission rate (WVTR) values up to 10-3 to 10-4 g/m²/day/atmosphere to protect silicon coatings.

Additional considerations
Fluoropolymer films used in front sheets have inherent fire-resistance and are seen as better than combustible films in not only meeting building code requirements, but also in allowing modules to be assimilated into building-integrated photovoltaic (BIPV) construction. Lastly, the toughness of polymeric front sheets provides a key benefit in reducing the loss of material and associated cost that occurs during manufacturing, transportation, and installation.

To meet such demands, new 50- to 100-micron films have been developed with corona treatments and special coatings. They are now easily accessible as a result of the strong efforts and partnerships developed between resin suppliers and filmmakers, who are active in packaging and specialty films. These unique film solutions give PV module manufacturers new opportunities to design innovative panels, produce roll-to-roll modules, and significantly reduce the weight of PV front sheets by 0.08 to 0.17 kilogram/meters squared (kg/m²).

These solutions have even been successfully tested on the first prototype of the Solar Impulse (HB-SIA), a Swiss long-range solar-powered aircraft. They’ve also been replicated on the second prototype (HB-SIB).

It’s worth noting the potential limitations that should be addressed when switching from rigid glass to flexible film, however. These include the loss of stiffness and, in some cases, the increased risk of silicon cell breakage. Compensating measures might include reinforcement of the module back sheets or the frame.

With new film manufacturers strongly committed to producing custom and price competitive polymeric films, a new generation of lightweight PV panels are expected to launch in the near future. These new products will answer the growing market need for lightweight, high-performances modules—and for both crystalline silicon and inorganic thin films.

Philippe-Jacques Leng was appointed European sales development manager (films and foams) for Solvay Specialty Polymers in 2008, and in 2012 was named global business manager – films. He’s responsible for setting Solvay’s global market strategy for the films business and driving growth in the PV market.

Solvay Specialty Polymers
www.solvay.com