Investigation of Underperformance in Fielded N-type Monocrystalline Silicon Photovoltaic Modules

As photovoltaic (PV) modules continue to evolve, it is important to catch and understand the causes behind new failure modes. Herein, we study n-type monocrystalline silicon PV modules that have been fielded at a utility scale power plant for 5 years, all of which have already degraded to < 90% of the nameplate max power (Pmp). High resolution electroluminescence (EL) and photoluminescence (PL) imaging suggests multiple possible factors contributing to the modules' underperformance, including series resistance issues (Rs) and wafer non-uniformities. Dark lock-in thermography (DLIT) measurements on a selected module suggests two specific module strings have high Rs. We then tabbed out all 60 cells of the same module. Suns-Voc measurements confirm relatively higher Rs values for the cells in these two strings. Multi-irradiance IV scans show the largest underperformance at the cell level for these same cells. This implicates Rs, rather than wafer non-uniformity, to be the largest contributor to the cell and therefore module underperformance.