Luca Gnocchi
The adoption of crystalline silicon (c-Si) PV modules based on glass-glass (G-G) structures is gaining momentum because of the possibility to manufacture bifacial panels, collecting light from the rear side. Despite not being an optimal material, Ethylene Vinyl Acetate (EVA) has become the dominant encapsulant in the PV industry. In fact, upon exposure to UV at high temperatures in the presence of moisture, EVA generates acetic acid (HAc), leading to the corrosion of the metallic interconnects. This problematic may be even more pronounced in a non-permeable G-G structure, compared to a conventional glass-backsheet (G-BS) design, in which HAc can be partially degassed out through the breathable BS. Nevertheless, some industries would like to continue using EVA in the manufacturing of G-G modules, because of the lower cost, the much longer track record and easier processability of EVA compared to newer alternatives, such as polyolefns (POs). In this work, we try to answer the question whether it is possible to use EVA in the manufacturing of G-G modules. The experimental chapters focus primarily on study the long-term reliability, in particular:1. The sensitivity to water of different c-Si cell technologies (i.e. Aluminum Back Surface Field (Al-BSF), Passivated Emitter Rear Cell (PERC) and Silicon Heterojunction (SHJ)) exposed to damp-heat, where we highlight the good stability of the EVA, and the sensitivity to moisture of the SHJ solar cells.2. We propose a detailed microscopical model explaining the problematic of the SHJ to water. In reality, the presence of moisture (favored by the use of EVA) is only a contributing factor, as clearly indicated by the fact that the degradation in more pronounced in the presence of glass. Here we point at the role of a glass corrosion process and of NaOH in damaging the passivating properties of the cell. Either by reducing the water ingress (with an edge seal or a PO) or by replacing the glass type, the degradation phenomenon is not observed.3. The impact of EVA storage conditions on the lamination quality and ultraviolet (UV) degradation. If good polymer storage and handling practices are carefully respected, the results tend to suggest that EVA can still be a viable solution to encapsulate G-G PV modules, for deployment in geographical zones where the humidity levels are not so high during the year (i.e. temperate climates). On the contrary, if these conditions are not observed, or in the event of module operating in a hot-humid climate, we believe that this may affect the long-term performance of G-G modules encapsulated with EVA.4. An accelerated aging test for acetic acid corrosion (missing in the industry standards) is developed to probe wear-out and end-of-life behavior and facilitate screening of metallization, and interconnection technologies. Corrosion is in fact one of the main end-of-life failure modes in PV modules which use EVA. Here, SHJ cells with a low temperature silver paste-based interconnection outperformed cells with conventional solder-based interconnection (i.e. Al-BSF and PERC). The proposed corrosion test method can be optimized to match corrosion behavior observed in feld modules with shorter times than standard damp heat tests.These results give us a deeper insight on how to manufacture reliable and durable G-G modules, based on different cell technologies, and do not rule out the possibility to use EVA when targeting extended lifetimes of double glass panels.
EPFL2022
