Glass vs. Backsheet: Deconvoluting the Role of Moisture in Power Loss in Silicon Photovoltaics with Correlated Imaging during Accelerated Testing

Moisture ingress is an established issue for photovoltaic module durability. Durability studies that probe moisture effects typically evaluate performance losses at the module level, attributing global power losses to the overall humidity condition of the test environment while leaving local module behavior unknown. We leverage our recently published water reflectometry detection method and biased photoluminescence imaging to spatially correlate moisture content and cell performance over 2500 h of damp heat testing. These tests, carried out on glass-glass and glass-backsheet mini-modules at multiple temperatures and humidities, reveal two dominant modes of local cell performance loss - acute finger interruptions and cell-wide 'background' series resistance increase. Finger interruptions occur earliest in glass-glass modules and at dry conditions. We do not observe any signs of contact corrosion at the front side in either module types. Still, glass-backsheet modules show a unique increase in series resistance at high humidities, suggesting a mechanism at the back contact. Overall, our results point to the humidity level of installation climate as a driving variable in selection of the bottom module layer.