Yellow to brown discoloration

Yellow to brown discoloration ranges from light yellow to dark brown. It can show in the encapsulant over or between cells, along busbars and interconnects, or on either side of the backsheet. In glass/backsheet modules the browning usually sits in the middle of each cell, with a clear frame around the cell edges and along cracks. This clear frame comes from photobleaching and should not be mistaken for snail tracks. If the browning is even across the module, encapsulant discoloration is the likely cause. It reduces short-circuit current, so look for a slow loss of current in IV curves or monitoring data. If one cell is much darker than its neighbours, that cell ran hotter, possibly because it sits above the junction box. Check the junction box for a hot spot with an IR camera. Yellowed, browned or bubbled potting inside the junction box, or burn marks behind it, point to a junction box interconnection failure caused by heat or arcing. This can start a fire, so treat it as a safety issue. If the yellow, brown or black color is on the gridlines, solder bonds or ribbons rather than the encapsulant, suspect metallization corrosion. EL images may show dark cell edges, and white or colored powdery deposits may be present. Discoloration of the backsheet can come before backsheet cracking. During a visual inspection, check discolored backsheets for cracks and chalking.

<!-- review notes

• sentence 3: support check: partial - Passages say uniform discolouration is typical for glass/glass and that yellowing reduces Jsc, but do not state that even browning implies encapsulant cause or the diagnostic advice about slow current loss.
• sentence 7: support check: partial - Discolouration is stated as a precursor to backsheet cracking, but the passages say nothing about checking for cracks and chalking during visual inspection (chalking is tied to strong chalking, not specifically advised). -->