Dependence of adhesion on degradation mechanisms of ethylene co-vinyl acetate encapsulants over the lifetime of photovoltaic modules
The long-term reliability of encapsulation has a critical impact on the overall cost and effectiveness of photovoltaic modules. An often overlooked but important element of this reliability concerns adhesive degradation of the encapsulant, which results in delamination. This work critically examines the degradation mechanisms – deacetylation, β-scission, and hydrolytic depolymerization – primarily responsible for adhesion loss in ethylene co-vinyl acetate (EVA) encapsulants to provide evidence and clarity regarding the contribution of these mechanisms to the adhesive degradation over the lifetime of a module. EVA samples obtained from mini-modules exposed in two sets of controlled, accelerated conditions – with and without ultraviolet (UV) radiation – are characterized to find specific evidence for the proposed degradation mechanisms. The mini-modules were exposed up to 10,000 h, which represents up to 15–20 years in the field. Results affirm that initially, deacetylation dominates the degradation before scission takes over, followed by hydrolytic depolymerization. Additional analysis highlights the climacteric interdependency between deacetylation and hydrolytic depolymerization as it relates to eventual delamination, where deacetylation products catalyze interfacial hydrolysis reactions. Furthermore, effects occurring at a given interface are isolated, explicitly providing evidence for how these mechanisms are nonuniform throughout the thickness of the EVA. Results demonstrate that the EVA that interfaces with the glass as opposed to the cell experiences increased levels of degradation, explaining why this initially stronger interface becomes the preferential interface for delamination after longer periods of exposure. Additionally, elevated temperatures alone are shown to induce limited migration of sodium ions across the glass/EVA interface.