Educating customers and learning from them

PV modules are generally expected to generate power for more than 25 years. The generation of power has to be reliable as well as safe. In order to achieve this objective, the active material of the module, the semiconductor layers or cells, are embedded in a package of non-active materials such as glass and polymer materials. The non-active materials may well be considered the less critical parts of the module, but they are extremely important for safety and long-term performance of the module itself. The predominant structures of non-active materials for crystalline and polycrystalline modules comprise of glass, EVA (ethyl vinyl acetate) encapsulant and multilayered backsheet. Given that reliable long-term module performance is critical to the profitable operation of PV systems, system owners prefer modules that use the highest quality non-active materials to protect the active layer.
Like glass, the backsheet is responsible for weathering the elements. In general terms, the backsheet is like the skin of the module that has to protect it from scratches and damage and, most importantly, insulate it. Quite obviously, if the backsheet is damaged, dampness will permeate the material and produce a corrosion process.
If this occurs, the backsheet no longer provides electrical insulation and thus electrical shock from the frame becomes possible with obvious safety implications.
The traditional, old structure of a multilayer backsheet consisted of a polyester film (PET) for the inner layer, laminated between two layers of polyvinyl fluoride (PVF), more commonly known as DuPont’s Tedlar brand (TPT), or a Tedlar, PET, EVA primer (TPE).
PET was chosen technically for its excellent electrical insulation, low moisture permeation and high mechanical strength, and commercially because it is available in large volumes and relatively inexpensive compared with other engineered polymers. PET was only restricted in the past to a role of passive central layer due to standard grades having low hydrolysis and UV resistance. Coveme predicted that the lack of abundance of Tedlar would be the crucial limiting factor in the development of the PV backsheet market.
This led Coveme to explore alternative materials such as fluoropolymer coatings and films. However, the development of high-grade polyester film was the only solution that utilized an abundantly available material while achieving high performance. Venturing away from Tedlar was the key strategic decision to facilitate growth as fast as the market. Thus, Coveme became the first company to successfully introduce a real alternative to TPT/TPE with the development of dyMat PYE and dyMat PYE3000.
In order to develop the optimal PET-based backsheet, Coveme investigated three key areas:

• Hydrolysis-resistant PET films

• UV outer layer stability

• Adhesive performance

Fortunately, the growth of the PV market has been matched by the development of polyester films which are much more resistant to hydrolysis and UV degradation.

Hydrolysis resistant PET films

Chemistry dictates that standard PET film grades hydrolyze and therefore lose their mechanical strength when exposed to damp heat test (DHT) conditions (85 degree Celsius/85 percent relative humidity). However, significant advances in manufacturing techniques have allowed the development of highly hydrolysis-resistant PET films with great plastic properties even after 3,000 hours of DHT. Under rigorous testing, the dyMat PYE3000 underwent over 3,000 hours of DHT and still retained an elongation to break (ETB) greater than 30 percent. The critical point for cracking is below ten percent ETB (this point is normally reached in around 1,200 hours in standard grade PET) (see graph PYE 3000 inner layer performance below).

UV outer layer stability

Unstabilized polyester films are chemically prone to UV degradation due to the presence of absorbing groups in the molecular chain. Off-the-shelf polyester films are unable to comfortably pass standard PV qualification tests, but significant advances in polyester chemistry and polyester film production engineering have allowed the development and commercialization of highly UV durable polyester films. It is known that UV light induced degradation will cause:

• Yellowness increase (formation of new light absorbing chemical species)

• Haze increase (clear films)

• Gloss decrease (surface roughens)

• Light transmittance decrease (more absorption and scattering)

• Mechanical properties decrease (reduction in ETB percent)

The UV durability of a material is usually evaluated by accelerated weathering techniques. A Xenon arc lamp in an Atlas Ci5000 Weather-Ometer (calibrated at 0.55 W/m2/nm at 340 nanometers (nm), according to ISO 4892-2) was used to age UV-stabilized polyester grades. Results are shown for the main properties.
The Weather-Ometer data shows the effect of UV-stabilizing a white polyester film. The chart (Retention of ETB) is for two films: a 50 µm (micrometer) thick standard white film (unstabilized); and a stabilized 50 µm film used for the outer layer of a typical backsheet. After 10,600 hours, the stabilized film shows excellent retention of ETB and very little increase in yellowness. With the lamp at this intensity, 1,637 hours in the Weather-Ometer is equal to one year in direct Florida sunshine. Keeping in mind that the backsheet is not exposed directly to sunlight, most of the irradiation will typically be by reflection. Most surfaces will typically have ten to 20 percent reflectance in the wavelength range that causes degradation to PET. Therefore, assuming 15 percent reflectance, 25 years in direct Florida sunshine would be equal to 6,139 hours in the Weather-Ometer (25 x 1,637 x 0.15 = 6,139). Thus, the 10,000 hour sample safely simulates more than 25 years in the sun.
These UV durability results required Coveme to educate the customer base. The PV market was accustomed to using Tedlar for the outer layer due to its well-known quality as a good UV barrier. Replacing it with a modified and high performing polyester film required a great deal of know-how sharing with customers. It was fundamental that Coveme made clear what level of UV protection was required, by calibrating the level of protection to the effective amount of UV radiation hitting the backsheet. For some components manufacturers, it may be tempting to use high-energy light sources to accelerate aging and therefore reduce the screening time for new materials. However any move away from true sunlight will give artificial results and the further away from sunlight the less realistic the results will be. Radiation with a wavelength (?) less than 300 nm, which occurs very little at sea level, will unnaturally age polymers and test results will not correlate well with outside tests. UVB aging is still used as a cheap but rather unreliable method to predict polymer lifetime. UVA also only emits in a specific portion of the UV light range and can be highly misleading since photo-oxidation of the initially generated yellow species happens upon exposure to the 370 to 430 nm range, a process known as photo bleaching. Consequently, PET can appear more yellow after UVA lamp testing, despite not much degradation having occurred. On closer inspection, these testing methods can severely misrepresent how a material would perform when exposed to real sunlight for 25 years.

Adhesive performance

Achieving polyester materials able to withstand several thousand hours of DHT was a great step forward, but it was only half the battle. The next step was designing an adhesive system that retained its adhesion properties as long as the entire life of the polyester materials. The market wanted a material with adhesion beyond the 1,000 and 2,000 hours of DHT. Coveme went the extra mile and got the entire backsheet to withstand 3,000 hours of DHT. It goes without saying that material benchmarking and internal testing are essential for any backsheet manufacturer aiming to provide consistently high quality. The development of our own internal testing was instrumental in achieving a leading position in the market. As a matter of fact, considerable differences in material aging can and will arise even under similar weather conditions. To offset this uncertainty, our materials are tested and benchmarked with targets that far exceed the industry averages. We know that our customers will conduct their own test before they choose a specific backsheet. The best thing we can do is always to listen to their needs and implement with the highest standard. The involvement of our R&D Lab in any technical demonstration has always been one of Coveme’s strong points in an industry where it was not common to share technical details and inside information.

What’s next

Over the last few years, numerous new polymeric materials have emerged for PV applications. This is a positive phenomenon, as the new materials will generate more choices with lower costs and higher performance. However, some concerns need to be addressed.
Accelerated stress test: Current procedures outlined by IEC and UL and other certifying groups, require several months for completion. Most of the PV module component manufacturers normally double the standards (e.g. 2,000 hours DHT instead of the 1,000 hour minimum requirement) or, as in Coveme’s dyMAT PYE3000, 3,000 hours of DHT. This generates a lengthy procedure that contributes to the delayed introduction of materials to the market. In order to shorten accelerated aging, many companies are now using the highly accelerated stress test (HAST). The HAST procedure is yet to be standardized and correlated with the DHT and real life outdoor performance. Specifically, Coveme’s development largely leveraged our experience in the pressure cooker test (closed chamber at 121 degrees Celsius and 100 percent relative humidity).
Thermal stability: Relative thermal index (RTI) is a measure of the thermal stability of a polymeric material and helps determine whether a material is suitable for continuous use at specific temperatures. Given that the backsheet is a multilayer structure and the current 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. Currently, certifying agencies offer different opinions on this matter. In addition, RTI is an exceedingly long test (over one year). This serves to slow the uptake of new materials. RTI is normally used where the operating temperature is well known and fairly constant (which is not the case for PV). PV modules rarely operate at the temperature measured in the temperature test.
Partial discharge test (PDT): The partial discharge test comes from the insulation industry and is typically used to evaluate the insulation on windings in transformers. Partial discharge is supposed to help predict wear of the insulation material after long-term exposure to electric fields. IEC 61730 requires the backsheet material to have a partial discharge rating for the highest system voltage the module will be rated for (typically 600 volts in the US and 1000 volts in Europe). The PDT is a fundamental test to pre-qualify the backsheet structure and materials. Some module manufacturers would like to remove it from IEC 61730 in order to introduce thinner materials but this is a vital safety measure that should not be compromised.

Conclusion

In conclusion, it seems appropriate to state the obvious: price/performance tradeoffs will drive material changes. A PV manufacturer will only adopt new materials if this is able to generate a direct improvement in the module efficiency, assuming manufacturability and logistic concerns are under control. Among the non-active materials, backsheet formulation is changing faster than the other material classes. For this reason, PV manufacturers will have to keep thoroughly testing the relevant components of their modules and possibly add new testing procedures to always be at the forefront. R&D Labs like Coveme’s are always at the customers’ disposal for new material testing and evaluation.
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