Reproducing Bimodal Series Resistance Degradation in Fielded Crystalline Silicon Photovoltaic Modules

This case study investigates mono-crystalline silicon modules from a utility site flagged as underperforming. Field characterizations revealed suspected damp heat induced series resistance driving module degradation. Select modules were removed from the field and incremental damp heat accelerated testing was conducted. A bimodal distribution of cell degradation became apparent. Cross-sectional scanning electron microscopy, energy-dispersive X-ray spectroscopy, and scanning spreading resistance microscopy reveal the most significant differences between the two are grid finger width and contact resistance. We discuss the reliability implications of re-test requirements in IEC 61215 for bill-of-material changes and how they may have mitigated degradation at this site.