The rapid growth of the photovoltaic (PV) industry over the last several years led to the development and introduction of numerous new materials for PV applications. This is a very positive phenomenon as new materials allow for more choices, higher performance, and cost reduction of final products. On the other hand, development, introduction to the market and manufacturing of new materials is a challenging process. One of the challenges back sheet manufacturers are facing is the certification process. Since these new polymeric materials do not have a long history of performance in PV applications, extensive testing to ensure safety and reliability of these materials is required. Current IEC and UL standards outline testing procedures for polymeric materials for PV applications and are used as guidelines. However, these standards have shortcomings, some of which are outlined below.

Time required for completing the testing. Current procedures such as damp heat, UV exposure and other tests require 2-3 months for completion. Although current standards necessitate only 1000 hours of exposure to damp heat, back sheet and module manufacturers extend this procedure to 2000 hours or longer to ensure the material performs adequately upon aging. Such lengthy procedures result in the delayed introduction of materials to the PV market. Most companies, in an attempt to "accelerate" accelerated aging utilize alternative test procedures. For example, HAST (highly accelerated stress test) has become popular among PV module component manufacturers and PV module manufacturers. However, the HAST procedure needs to be standardized and correlated with damp heat and outdoor performance.

Interpretation of the requirements. Some standards do not clearly define the performance expectations of polymeric materials upon completion of the tests. For example, back sheet manufacturers perform damp heat testing, measure and report adhesion values between the layers of the back sheet or between the back sheet and the encapsulant as a function of exposure to damp heat. Currently, the damp heat test protocol does not indicate what results are expected at the end of the test.

There is limited consensus between different parties, including certifying agencies, in the interpretation of the requirements described in the IEC and UL standards. For example, UL 746 C states that only materials directly exposed to sunlight have to be tested with respect to UV stability. This statement raises a lot of questions. Is this requirement applicable to back sheets? How should the test be performed? Should it be done by a certifying agency or will internal test results suffice?

Relative thermal index (RTI) value requirement is even more complicated. RTI is a measure of the thermal stability of the polymeric material and determines whether the material is suitable for continuous use at a certain temperature. Usually, back sheets are multilayer structures. As of now, the standard states that the RTI value of one layer can be assigned to the full construction. It is unclear which value should be assigned to the full construction in the case where individual layers have different RTI values. Certifying agencies have different opinions on this matter.

Flammability is another key property that needs to be defined clearly. The PV community is growing beyond just being a source of power for small homes or communities. It is reaching proportions as a serious alternate energy source to power the electric grid. Safety and reliability are the primary requirements. With that said, tests have to be defined taking into account limitations of polymeric materials without compromising safety. Back sheet manufacturers in particular, and module manufacturers in general, are under serious pressure to reduce cost materials used in a manner to achieve grid parity and certain polymeric materials provide the path to achieve this goal.

Another challenge is to ensure the continuous supply of raw materials to meet the ever-growing demand of the PV Industry. Most of these materials are polymeric-based and are subjected to the same slew of certification tests, delaying the process of introduction to the PV market. Polymeric materials provide the required performance and properties for use in the manufacture of PV modules, but are subject to tests with inadequately defined or arbitrary performance requirements, causing confusion and delay in approval of the same for use in the manufacture of components for PV modules.

The PV community realizes these limitations and is working on the new IEC document that will address these and other concerns. However, the timetable for the new standards that will bring relief to the highlighted challenges is a couple of years from completion. The current exponential growth of the PV industry is placing significant strain on the PV module and component manufacturers alike. A short term solution is required to screen and qualify the much needed materials to continue supporting the innovative efforts of the PV industry.

Marina Temchenko received her Masters in polymer chemistry at Moscow U. and is Sr. Scientist at Madico, Inc., 64 Industrial Pkwy, Woburn, MA 01801 USA; ph.: 781-935-7850; mtemchenko@madico.com

Vankatesh Natarajan received his MS in engineering at U. Mass Lowell and is Sr. Quality Manager at Madico, Inc.

Dave Avison received his Masters in polymer chemistry at U. Mass Lowell and is Director of the Core Technology Group at Madico, Inc.