Discoloration happens when the polymer encapsulant, usually EVA, or the backsheet breaks down and turns light yellow to dark brown. The colour is often patchy: in glass/backsheet modules it tends to sit in the middle of cells with clear frames at the edges, while in glass/glass modules it is mostly even. Clear rings around cell edges or along cracks are photobleaching, and they should not be confused with snail tracks. Heat and UV light acting on poorly stabilised encapsulant or poorly laminated modules are the main drivers. Confirm it by visual inspection, then check the electrical impact with IV curve tracing and infrared thermography. It starts as a cosmetic issue and causes slow current and power loss, typically about 0.5 percent per year and up to 1 percent per year in hot, humid or moderate climates. Watch closely when one cell is much darker than its neighbours, because severe local discoloration can switch on a bypass diode. A discoloured UV-sensitive backsheet can later crack.
<!-- review notes
affectsc1 drafted · lead Encapsulant
sec=h58 h=98e749035673sec=h24 h=017575ddabf4sec=6.2.1 h=2cb68650cddfsec=h58 h=98e749035673sec=h24 h=017575ddabf4presents_asc3 drafted · lead Yellow to brown discoloration
Light yellow to dark brown colour over or between cells, along busbars or interconnects, or on the backsheet; often patchy, with clear frames around cell edges in glass/backsheet modules.
sec=h58 h=98e749035673sec=h24 h=017575ddabf4c4 verified · fact Encapsulant photobleaching
Oxygen bleaches the chromophores, leaving a clear ring around the edge of a cell or along a cell crack.
sec=h58 h=98e749035673sec=h24 h=017575ddabf4sec=6.2.1 h=2cb68650cddfverified by: joshuahamsaverified on: 2026-10-20
c5 drafted · lead Gradual performance decline
Module current and power fall only after the discoloration is already visible.
sec=h58 h=98e749035673sec=h24 h=017575ddabf4sec=6.2.1 h=2cb68650cddfc6 drafted · lead Bypassed substring
Only when the discoloration is very severe and confined to a single cell.
sec=h58 h=98e749035673sec=h24 h=017575ddabf4sec=6.2.1 h=2cb68650cddfdetected_byc7 drafted · lead Visual inspection
sec=h58 h=98e749035673sec=h24 h=017575ddabf4c8 drafted · lead IV curve tracing
Listed as a secondary detection method.
sec=h58 h=98e749035673sec=h24 h=017575ddabf4c9 drafted · lead Infrared thermography
Listed as a secondary detection method.
sec=h58 h=98e749035673sec=h24 h=017575ddabf4caused_byc10 drafted · lead UV exposure
Heat and UV acting on EVA; photo-oxidation produces discoloration.
sec=h58 h=98e749035673sec=h24 h=017575ddabf4sec=6.2.1 h=2cb68650cddftbl=12.r2 h=2a0a2a40eb18c11 drafted · lead High operating temperature
Thermo-oxidation causes discoloration. A single dark cell suggests that cell ran hotter than its neighbours.
tbl=12.r1 h=f90c82d02f74sec=h58 h=98e749035673sec=h24 h=017575ddabf4sec=6.2.1 h=2cb68650cddfc12 drafted · lead Low-quality material
Additives that are poorly chosen or at the wrong concentration, or UV absorber that is used up over time.
sec=h58 h=98e749035673sec=h24 h=017575ddabf4sec=6.2.1 h=2cb68650cddfc13 verified · fact Manufacturing defect
Lamination that is inadequate or incomplete.
sec=h58 h=98e749035673sec=h24 h=017575ddabf4verified by: joshuahamsaverified on: 2026-10-20
leads_toc14 drafted · lead Backsheet Cracking
Discoloration of UV-sensitive backsheets can lead to loss of elasticity and to cracking under thermomechanical stress.
sec=h58 h=98e749035673sec=h24 h=017575ddabf4c15 drafted · lead Severe Metallization Corrosion
The sources report a correlation with acetic acid generation and accompanying corrosion, not a direct cause.
sec=h58 h=98e749035673sec=h24 h=017575ddabf4sec=6.2.1 h=2cb68650cddfmitigated_byc16 drafted · lead Stabilized encapsulant formulation
Better EVA stabilisation with UV absorbers and thermal stabilizers has largely solved the problem.
sec=h58 h=98e749035673sec=h24 h=017575ddabf4degradation_ratec17 drafted · lead nominal 0.5 PERCENT-PER-YR
Conditions: Typical mean yearly degradation from yellowing
sec=h58 h=98e749035673sec=h24 h=017575ddabf4c18 verified · fact ≤ 1 PERCENT-PER-YR
Conditions: Yellowing in hot and humid or moderate climates
sec=h58 h=98e749035673sec=h24 h=017575ddabf4verified by: joshuahamsaverified on: 2026-10-20
c19 drafted · lead ≤ 0.5 PERCENT-PER-YR
Conditions: Expected share of EVA discoloration in the roughly 0.8%/a degradation commonly seen for Si modules
sec=6.2.1 h=2cb68650cddftypical_power_lossc20 drafted · lead nominal 10 PERCENT
Conditions: Approximate total loss of module performance when discoloration is severe
sec=6.2.1 h=2cb68650cddfreview note: support check: partial - Passage says ~10% total loss appears with severe discolouration, but 'typical power loss' framing is stronger than the source's approximate statement.
leads_to c11leads_to c24leads_to c21caused_by c6