01 February 2011
Polyolefins are nonpolar, nonporous thermoplastic polymers that offer easy processing, elasticity, toughness, thermal stability, moisture-resistance and recyclability. These qualities have made polyolefins the materials of choice for a virtually endless array of products – from medical goods and flexible packaging to footwear, foams, and adhesive and protective films. Now these same performance benefits are helping reduce cost/watt/area of solar modules, where polyolefins are being used, among other applications, as encapsulant films that help lower total system costs via improved processing and enhanced module performance.
Several challenges have limited the overall effectiveness of today’s photovoltaic encapsulants.
Historically formulated with products such as ethylene vinyl acetate (EVA), solar encapsulant films work as a barrier to environmental attack while maintaining the integrity of the solar cell electrode. Module manufacturers expect encapsulant films to have: excellent optics; adhesion to a variety of substrates; chemical and thermal stability; and ready processability in thermal lamination processes.
There are several challenges that have limited the overall effectiveness of today’s photovoltaic encapsulants. EVA films can absorb moisture over time, releasing acetic acid that attacks the transparency coating of the photovoltaic cell and circuitry. The hydrolysis of EVA films has also been shown to result in delamination at the module edge and along the wires, as well as yellowing – both of which decrease cell reliability and efficiency. In addition, EVAs need to be cured during module manufacturing, which results in relatively long cycle times in the vacuum lamination process.
Now, a new range of polyolefin encapsulants is showing great promise in helping module manufacturers overcome these hurdles. The Dow Chemical Company, for example, recently introduced Enlight Polyolefin Encapsulant Films. (For additional information about ENLIGHT Polyolefin Encapsulant Films, please visit the Enlight website or Dow’s Customer Information Group at .) According to Dow research, which was conducted with top-tier module manufacturers and other industry organizations, polyolefin encapsulant films offer a number of advantages compared to EVA encapsulants.
- Increased stability: Modules made with polyolefin films can demonstrate excellent stability in various environments, leading to significantly improved reliability levels and extended service life. The Glass Transition Temperature of most polyolefin encapsulant films is much lower than that of EVA films, allowing them to offer improved cold temperature performance. The stability of polyolefin encapsulant film also allows it to maintain module reliability levels over extended periods of exposure to damp heat—a property that can significantly extend service life.
- Processing efficiency: Since polyolefin films do not require crosslinking, lamination times can be reduced up to 30% compared to EVA, resulting in increased yield and capacity.
- Moisture resistance provides long-lasting protection: In EVA films, corrosion caused by the release of acetic acid over time results in the eventual reduction in module performance. With polyolefin technology, damage to equipment and modules is reduced because the film does not produce the corrosion-causing acetic acid.
- Improved electrical performance: Polyolefin films can offer the ability to insulate the module and maintain cell reliability with high volume resistivity levels. Higher volume resistivity allows the module to best conduct the electrical current and enhances and maintains efficiency levels. As a result, electrical performance is enhanced and service life can be extended.
Customizing a PO encapsulant
Unlike EVA encapsulants, polyolefin chemistry also offers broader design flexibility, allowing the materials to be custom-tailored to fit exact module specifications and needs. Polyolefin chemistry enables encapsulant films to be made with a broad range of optics, moisture barrier and modulus, making them well suited for current c-Si solar modules and emerging thin-film applications.
About the Authors
Brij Sinha is Global Strategic Marketing Manager for Photovoltaics, Dow Packaging & Converting Business. Mark Hofius is Senior R&D Leader, Dow Packaging & Converting Business.
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