Crosslinking and post‐crosslinking of ethylene vinyl acetate in photovoltaic modules

Abstract

Ethylene vinyl acetate (EVA) is the dominating material for the encapsulation of solar cells. A better understanding of the crosslinking reaction progress during PV module lamination could lead to promising approaches for shortening of PV module lamination times but also for optimization of the EVA formulation. Therefore, the main aim of this study is to investigate the crosslinking behavior of EVA but also for optimization potentials of the EVA formulation. Currently, a degree of crosslinking higher than 70% obtained from Soxhlet extraction, is used as quality control standard in PV industry. Thermomechanical properties of the investigated EVA films demonstrate a sufficient state of crosslinking already after 5 min, which corresponds to a Soxhlet value of around 50%. Nevertheless, the effect of the remaining, still reactive peroxide crosslinker under service relevant conditions cannot be neglected. Therefore, the behavior of mini‐modules manufactured at different lamination times and stored under various aging conditions is investigated. EVA not fully cured during lamination are undergoing postlamination crosslinking. At the same time, remaining active crosslinker causes discoloration at soldering ribbons after accelerated aging. The crosslinking time in the lamination process may be reduced to ≥6 min, compromising between high throughput in production and the need of avoiding degradation. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44912.

The Influence of the EVA Film Aging on the Degradation Behavior of PV Modules Under High Voltage Bias in Wet Conditions Followed by Electroluminescence

“…Presumably, these leftovers of peroxides react with silver which leads into the corrosion of finger and busbar conduction lines as observed in modules Maged and Mexpired. This is in good agreement with other studies published in literature, where several authors reported a correlation between poorly crosslinked EVA and discoloration of soldering ribbons as well as corrosion of the silver gridlines [24]- [27]. The observed degradation effects have been explained by the high amount of still reactive peroxides in the modules which can react either directly with the surface of the metallic PV module components or with degradation products of the EVA encapsulant like acetic acid and subsequently lead to a corrosion of the soldering material or the silver grid lines.…”

Properties and degradation behaviour of polyolefin encapsulants for photovoltaic modules

“…For EVA and POE, an exotherm peak starting around 125 C due to the decomposition of the cross-linking agent can be seen before lamination. 8,9 The In the first TMA heating run, all three types of encapsulants showed thermal expansion in both directions (machine direction MD and cross direction CD) indicating biaxial orientation, which is common in solar applications since it leads to better mechanical, optical and barrier properties. 52 However, for all test modules, yellowing was observed after DH test and the strongest for the material stacks using TPO as encapsulant (see Figure 7).…”

“…5 To achieve thermal and thermo-mechanical stability, the material is equipped with curing agents (peroxides) for chemical cross-linking to take place during the lamination process, where a three-dimensional polymer network is formed. [8][9][10][11][12] After this time-and energy-consuming module lamination process, it also leaves reactive peroxides in the PV module, which can influence or accelerate certain degradation modes. 9,13,14 Another drawback of EVA is the formation of acetic acid during chemical degradation of EVA, 5,15 which also accelerates the oxidation process of EVA.…”

“…[8][9][10][11][12] After this time-and energy-consuming module lamination process, it also leaves reactive peroxides in the PV module, which can influence or accelerate certain degradation modes. 9,13,14 Another drawback of EVA is the formation of acetic acid during chemical degradation of EVA, 5,15 which also accelerates the oxidation process of EVA. [16][17][18] Due to the impermeability of the adjacent layers (glass front-sheet and solar cells) as well as the gas barrier of backsheets, 19 the usually very volatile acetic acid cannot leave the PV module that easily and is enriched in the EVA encapsulant layers within the PV modules.…”

“…Temperature scans were performed at uncured samples between room temperature and 200 C at a heating rate of 3 K/min. 8,9 The thermal expansion behaviour was characterized using a at 1164 cm −1 , which can be attributed to a butyl acrylate side group. 48 The lower intensity of the C O peak indicates that POE has a lower comonomer content than EVA.…”