They have known the harmful effects of the UV radiation on organic compounds. The same way, the UV radiation affects also the chemical stability of the organic polymeric materials. In any case the level of harm depends on the specific energy sensitivity of the organic groups present in the molecules.
The Ethylene Vinyl Acetate (EVA) is the preferred polymer material that has been used as part of the photovoltaic modules because of its optical transparency, good adhesion and “easy” processing properties. These three properties have been the key reasons why EVA films have been and are still today extensively used for the construction of almost all types of photovoltaic devices.
The sensitivity of polymeric materials to radiation is an old and well-known problem in Polymer Chemistry. The EVA polymers are not an exception to that. In the specific case of the photovoltaic modules, this problem is extremely important because the EVA films might loose some of their critical performance properties, mainly their optical and the structural ones.
All the different PV EVA films available in the market are used as a joint interlayer between the glass and the silicon cells or the thin film structures. They are good PV interlayers due to their transparency which is critical for the module performance. But being optically so transparent they are not 100% transparent to the UV radiation. They absorb part of this radiation.
For that reason, EVA compounds should be conveniently protected against the most harmful wavelengths of solar radiation. As a consequence Non protected at all or Non well protected EVA films will suffer from degradation processes after time. There is no doubt that this is going to happen for sure. It is just a matter of time that once more “mother nature” will take the lead over us and our wishes. Our challenge, as Chemists, Engineers and Physics professionals, is to enhance the positive effects of the radiation, to make them more efficient, and to minimize the negative ones, the premature UV aging, so that the solar panels last longer.
Increasing their lifetime as well as enhancing their performance are two ways of increasing the market value of the panels. We know that by playing with these two technical parameters we have some control over the value of the modules. According to the equation Value = Benefit/Price we can easily understand that increasing the Benefit (time of operation and/or energy yield), we will be able to increase the value of the modules.
The solar radiation of wavelength between the 400-200 nm interacts with the chemical structure of the EVA, mainly with their carbonyl groups, and if these films are not well “UV protected” some “harmful degradation compounds” might be unexpectedly generated from the own EVA molecules. One of the most dangerous compounds that can be generated is the acetic acid that, when combined with other chemical radicals, might exponentially increase its “damaging efficiency” inside the module. These radiation generated sub-products are responsible for the uncontrolled chemical attacks to the other critical components of the photovoltaic modules, destroying its mechanical structure and its functionality. In most of the cases, the degradation of EVA can be detected on the module just by observing its transparency changes turning to yellow.
In fact, yellowing is a visible sign warning that the modules are reducing their electrical and properties, such as tensile, elongation and adhesion of the different parts, normally resulting in module delamination. Once the module starts losing its transparency normally the yellowing process speeds up dramatically.
For the electrical performance of the modules, at some level, the acid is not fatal to the electrical yield of most silicon-based solar modules, which might technically be still operative but yielding lower power, for thin-film technology, in particular, copper-indium-gallium-(di)selenide (CIGS), however, any acidic generated is devastating.
All this happens because the vinyl acetate groups of EVA chemical structures are prone to suffer the chemical attacks of the sub- products that are generated by the effect of radiation degradation of the EVA. Different chemical mechanisms have been proposed in the bibliography during the last years to explain Why EVA encapsulants turn yellow. Basic questions (the ones that go for basic knowledge) like which are the chemical compounds that are responsible for the yellowing? How are they formed? and the most important and practical ones: How harmful are the yellow species formed? What are the radical compounds formed together with the “yellow indicator”? What is the best way to block them?, have clustered the attention of the technical community around PV encapsulants.
Not surprisingly some of the answers to these questions are already included in the added value of every PV EVA sold in the market. The answers are part of the Know-How developed by each one of the encapsulant manufacturers. This know-how is present in the formulation and processing of the EVA. All these factors are somehow reflected in the price of the film.
In an effort to minimize or avoid this problem NovoGenio has worked to improve the UV resistance of our EVA base grade, the NovoSolar® FC (see the absorbance ranges of different EVA films in the graph below), at the time we developed new encapsulants, that came out to the market as the grades NovoSolar®PLX+ and NovoSolar® CL.
Basically these new Optical films have turned out to be chemically much more stable to the UV radiation than EVA. Our NovoSolar® non-EVA grades have been conveniently reinforced and protected against UV radiation to help improve the yields and long term performance of the new generations of photovoltaic modules. These new encapsulants are ready for the construction of the new generations of photovoltaic encapsulants.
In order to confirm these improvements, we have compared the yellowing of our EVA encapsulant grade with he NovoSolar® PLX+ and CL grades. No need to say that the comparison studies were performed under the same conditions. We should not forget that our EVA grade has been certified and considered a high performance material in the PV sector. So under our terms, we were using as a benchmark an already excellent market PV reference.
The following graphs show the reflection spectra from the different encapsulants and its calculated yellowness index (YI). The results of the YI clearly show that the CL and PLX+ do not turn yellow as the EVA does after 500 h and 1000 h DH (IEC 61215, 10.13). This test was performed at the Fraunhofer ISE at Freiburg, Germany.
Our work did not stop here, and in order to deeply analyze these results, some extended work was performed to compare the NovoSolar® FC (EVA based film) and the NovoSolar® PLX+ grade (Non EVA base film). The extended work was done analyzing the optical properties of both types of encapsulants under the effect of UV radiation through time. To get a summary of the results just click the below button.