Long-term field degradation of crystalline-silicon PV modules: insights from fifteen years of monitoring and forensic analysis

This study presents a 15-yr real-field performance and degradation analysis of photovoltaic modules in a Mediterranean climate. Three technologies—polycrystalline silicon (p-Si), monocrystalline silicon, and amorphous silicon—were monitored, with a detailed forensic focus on the most severely degraded p-Si systems. The p-Si modules exhibited an average degradation rate of 2.56%/year, significantly exceeding the manufacturer's warranty. Laboratory flash tests confirmed power losses of −33% to −70% over fifteen years. A multi-method diagnostic approach, integrating electroluminescence, ultraviolet fluorescence, lock-in thermography, and material forensics, identified two primary synergistic degradation pathways. The first involves optical losses from encapsulant yellowing, reducing current uniformly. The second, and more critical, is a severe increase in series resistance caused by interconnect corrosion and delamination, which drastically reduces the fill factor. Microcracks, while widespread, were a secondary factor. The study unequivocally links the degradation to moisture, oxygen, and light exposure, as evidenced by localized protection under a metal-backed nameplate. These findings underscore the critical discrepancy between accelerated testing and field aging, highlighting the necessity of long-term monitoring for accurate lifetime predictions and the development of more durable module materials and designs.