Solar Photovoltaic Module Degradation from Electro-Migration

摘要

This report discusses the effects of outdoor weathering on both encapsulated and unencapsulated solar photovoltaic modules. Degradation modes that result from exposure in a terrestrial environment and which influence long-term module performance are identified. Two modules, stripped of their protective RTV coating, and a third encapsulated module were placed in the Lincoln Laboratory Rooftop Test Bed in the open-circuit mode and monitored for five months. Monthly visual observations and power measurements were made. The encapsulated module showed no evidence of degradation during the five months of this investigation. The unencapsulated modules showed signs of both physical and electrical degradation. Auger analysis of these unencapsulated cells disclosed two potential degradation modes: first, the migration of Sn between positive and negative surfaces on the same cell; and second, the migration of Sn from the cell's negative grid (when this negative grid is adjacent to a cell row at lower potential or the metal module edge). This migrating Sn, combined with the SiOx antireflective film, altered the coating structure and appearance. Both degradation modes depend on the presence of moisture, and will thus reduce module peak power by 8 to 11 percent. The encapsulated module power remained relatively unchanged. The degradation resulting from moisture contacting the Si cells has the potential to eventually render a module inoperative. The prevention of moisture penetration and entrapment will significantly contribute to module longevity. (ERA citation 05:023805)