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Considerable degradation has been observed in negatively-biased, framed a-Si:H thin-film photovoltaic (PV) modules fabricated with earlier generation fluorine-doped tin oxide (SnO2:F) transparent conducting oxide (TCO) layers on superstrate glass. These modules were tested under high-voltage (HV) bias in a hot and humid climate and leakage currents (LC) from module frames to ground were monitored. Modules negatively-biased at -600 V, -300 V and -150 V showed clear signs of delamination after 8, 15 and 27 months respectively. The adhesional strength was completely lost in the damaged area. An a-Si:H cell in the degraded region delaminated entirely from the superstrate glass and transferred itself on to the bottom glass/EVA surface, then cracked and curled. LC values from support to ground in new, unframed laminates fabricated with an improved SnO2:F TCO layer were ∼100 times lower under a -600 V bias in the same hot and humid environment.