Non-uniform EL pattern

ID: symptom:el-nonuniform-pattern
IRI: https://solarlattice.org/kb/symptom/el-nonuniform-pattern
Status: active
Also known as: patchwork pattern

A non-uniform EL pattern is cell-to-cell or area-to-area variation in luminescence across a module, seen in electroluminescence images taken in the dark or at night while the module is forward-biased with a DC supply. In crystalline silicon modules, a patchwork pattern that grows over time can sometimes indicate ongoing light-induced degradation (LID or LeTID), even though LID itself leaves no visible defect. LID mainly shows as a drop in STC power, short-circuit current and open-circuit voltage early in the system's life, so confirm it with IV curves or flash tests rather than with EL alone. Potential-induced degradation can also look patchy, but its typical sign is dark cells near the module frame, clearest at low current bias, and it is worse in modules near one pole of the string. Bright and dark areas inside cells, or a single dim cell, can instead come from production problems such as uneven firing or a poor busbar connection. A single EL image taken at one bias current often cannot identify the cause, so take images at two bias currents and combine them with IR thermography or photoluminescence. In the field, use IR thermography first to find the anomalous modules, then do EL on site or in a lab, and send suspected underperformance to an accredited test laboratory for confirmation.

<!-- review notes

sentence 1: support check: partial - Passages cover EL imaging with forward-bias DC supply in the dark or at night and variation across module surfaces, but never define a non-uniform EL pattern as cell-to-cell or area-to-area variation.
sentence 3: support check: partial - Passages support the symptoms (drop in STC power, Isc, Voc) and verification with IV and flash testing, but do not say to use IV rather than EL alone; EL is even listed as a detection method.
sentence 6: support check: partial - Passages say a single-bias EL image may not identify the cause and that two bias currents combined with IR thermography, photoluminescence or LBIC can help, but do not prescribe it as an instruction beyond that. -->

Claims

`observed_as`

c1 drafted · lead Patchwork EL pattern

In crystalline silicon modules, EL images that grow less uniform over time (a patchwork pattern) can sometimes point to ongoing light-induced degradation (LID or LeTID).

Increasing non-uniformity of electroluminescence images (patchwork pattern) can in some cases highlight an ongoing degradation process.

Degradation and Failure Modes in New PV Technologies – Annex sec=h46 h=1ff4ba1a643e
Quantification of Technical Risks in PV Power Systems sec=h69 h=bdc3ce4bcd6c

c2 drafted · lead Dark cell edges in EL

This points to PID rather than LID. Cells near the module frame or boundary appear dark, most clearly at low current bias (one tenth of rated current).

Review on Infrared and Electroluminescence Imaging for PV Field Applications sec=3.3.1 h=1f95caadc75d
Degradation and Failure Modes in New PV Technologies – Annex tbl=15.r1 h=6e2cec50e730

review note: support check: partial - Passages say PID shows dark cells at module boundaries/near frame and is clearer at low bias (1/10 rated current), but do not compare it with LID or say such darkness 'points to PID rather than LID'.

c3 drafted · lead Lower EL intensity distribution

Modules with higher power loss tend to show a pixel-intensity histogram shifted toward lower EL intensity. This shift has not been clearly linked to any specific defect type.

Review on Infrared and Electroluminescence Imaging for PV Field Applications sec=3.1.1 h=06ce46a9a853

c4 drafted · lead Dim cell in EL image

A single cell that is much dimmer than the others can come from a poor busbar connection. Bright and dark areas within a cell can come from uneven temperature during cell firing in production.