Corrosion effects in bifacial crystalline silicon PV modules; interactions between metallization and encapsulation

This study addresses the influence of different encapsulation materials on performance losses in bifacial PV modules after extended damp heat testing. The widely used ethylene vinyl acetate (EVA) is compared to polyolefin elastomers (POE) and thermoplastic polyolefins (TPO). We show that modules based on n-type bifacial solar cells (tunnel oxide passivated contact, TOPCON) are more prone to degradation than p-type (Passivated emitter rear cell, PERC) based laminates. More specifically, the front side metallization of these n-type cells corrodes under extended damp heat testing, yielding a decreased fill factor in the I–V characteristics and the appearance of dark spots and dark areas in electroluminescence imaging. The newer generation of encapsulant materials (POE and TPO), slowly replacing EVA in the modules commercialized today, demonstrate an improved protection of the metallization. Apart from the nature of the encapsulant, the cell type and the presence or absence of glass cover plates are also shown to influence the metal grid corrosion. To better understand the mechanisms at play, post-mortem analysis was carried out on small sized degraded PV laminates, by drilling out circular samples ("coring") that could be analyzed in depth. The analyses revealed a weakened bond between the metallization and the silicon emitter, ultimately causing metal grid delamination. The proposed mechanism behind this delamination starts with the degradation of lead glass, which is part of the cell metallization grid and contains lead oxide (PbO). This compound is susceptible to dissolution in acidic environment.