Understanding and modeling the adhesive reliability and degradation of photovoltaic module encapsulants and other solar polymer films
Keywords: Materials Science and Engineering.

Within a photovoltaic (PV) module, the encapsulation serves a critical purpose in extending the useful life of this power source. Furthermore, the long-term reliability of encapsulation has a critical impact on the overall cost and effectiveness of photovoltaic modules. Through general insouciance in the field, the adhesive reliability and degradation, which results in delamination, is often overlooked. Ethylene co-vinyl acetate (EVA) encapsulants comprise the vast majority of those currently in use; thus, to address this failure pathway that has plagued modules for decades, the degradation mechanisms primarily responsible for adhesion loss in EVA -- deacetylation, β-scission, and hydrolytic depolymerization -- are critically examined to provide evidence and clarity regarding the contribution of these mechanisms to the adhesive degradation over the lifetime of a module. This investigation involved extensive, organized exposure of representative mini-modules to both natural field conditions and intensified accelerated aging. The accelerated exposures simulated 15-20 years of field aging over the course of just 10,000 hours, resulting in the corroboration of time dependencies for the degradation mechanisms and the discovery of a catalytic relationship between deacetylation and hydrolytic depolymerization. Critically, these results demonstrate that the EVA that interfaces with the glass as opposed to the cell experiences increased degradation, causing this initially stronger interface to become the adhesive weak link after longer exposures. In addition to the fundamental chemical degradation insight, EVA adhesion was characterized on mini-modules over the course of nearly 6 years of field aging and 10,000 hours of accelerated aging, providing crucial experimental adhesion data detailing unexpected trends over time. Instead of adhesion continuously decreasing with time, a distinct plateau emerges during the intermediate exposure periods (after 1 year in the field and after 1000 hours in a UV chamber) before precipitously dropping off. After connecting the microscale chemical changes with the macroscale adhesion strength, how EVA adhesion degrades and why delamination occurs become much clearer. Furthermore, this increased fundamental understanding in combination with the expansive network of experimental data facilitates a much more accurate modeling of adhesion trends through short, intermediate, and long-term exposures. Working off the framework of a previously developed adhesion model, we refined key model parameters and relationships regarding the rate of UV-radical formation and subsequent ß-scission, the rate and acceleration of hydrolytic depolymerization, and plasticity contribution in order to enable accurate predictions throughout exposure. Nevertheless, as highlighted through this work, EVA presents potential deficiencies with respect to delamination failure and serious challenges in extending the reliable lifetimes of solar modules. Accordingly, new materials and technologies are continuously being developed and explored as potential solutions and to advance the field overall. Inevitably, the long-term reliability of any new components remains unknown at first. Within this work, 2 key innovative material technologies/solutions are critically examined for reliability -- polyolefin elastomer (POE) encapsulants, which are the principal encapsulant replacements for EVA, and novel multiwire technologies that seek to improve on traditional busbar designs but introduce additional polymer foils into the lamination scheme. In both cases, the adhesive reliability of these polymer components had been largely unknown. Thus, this work provides a thorough examination of the adhesive reliability of both POE encapsulants and multiwire polymer foils on both the front and rear sides of bifacial solar modules. Established accelerated testing methodologies were employed to drive relevant degradation. Various thermomechanical and chemical characterization techniques were employed to understand the adhesive degradation trends. Results indicate that POE encapsulants maintain high adhesive stability throughout extensive accelerated aging, demonstrating their potential to help extend module lifetimes; however, the inclusion of the polymer foil in the multiwire assembly severely reduces the adhesion strength, resulting in the early formation of localized delamination zones and making major delamination failure a much greater possibility