Quick Facts
What Electroluminescence testing is
Electroluminescence (EL) is a non-destructive diagnostic technique used to identify defects in solar cells and modules that are invisible to the naked eye. The technique works by injecting a forward-bias current into the module in the dark; the cells emit near-infrared light proportional to local cell quality. A specialised camera captures the emission as a grayscale image where healthy cell areas appear bright and defective areas appear dark.
EL imaging reveals:
Microcracks: Hairline cracks from handling, transport, or thermal stress.
PID damage: Characteristic darkening at cell corners or along cell edges.
Cell-level hot spots: Damaged or shaded cell areas.
Inactive cell areas: Manufacturing defects or interconnect failures.
Bypass diode failures: Affected cell groups show specific patterns.
Many of these defects produce no immediate performance impact but degrade panels over time. EL testing catches them early, before they manifest as performance loss.
For solar plant operators, EL is one of the most valuable diagnostic tools, particularly for utility-scale plants where individual module inspection is otherwise impractical.
How EL works
The physical principle is straightforward.
When current flows through a solar cell in forward bias (opposite to normal operating direction), electrons and holes are injected into the silicon. They recombine, releasing energy as photons.
For silicon cells, the photon emission is in the near-infrared range, with peak wavelength around 1100 nm. The emission intensity is proportional to local cell quality: more electrons reaching that point, fewer captured by defects, more emission.
A specialised camera with near-infrared sensitivity captures the emission. The resulting image shows healthy areas as bright and defective areas as dark.
The process requires:
Forward current injection from a power supply (typically at the module’s STC current).
Complete darkness (any ambient light would saturate the camera).
A near-infrared camera with appropriate filters and sensitivity.
Software to enhance and analyse the captured images.
The test is purely passive observation; it does not damage the module.
Defects revealed by EL
Different defect types produce distinct EL patterns:
Microcracks: Appear as thin dark lines, often radiating from cell corners or edges. Caused by handling, transport, or thermal stress.
PID damage: Dark patterns starting at cell corners and spreading along cell edges. Caused by voltage-driven ionic migration.
Hot spots: Dark areas with sharp boundaries, often within a single cell. Caused by sustained shading or damaged cells.
Cell breakage: Sharp dark lines across cells. Caused by mechanical impact.
Solder bond failures: Specific patterns at the cell-to-ribbon junction. Caused by manufacturing defects or thermal stress.
Inactive cell areas: Large dark zones within otherwise normal cells. Caused by manufacturing defects.
Bypass diode patterns: When a diode has activated, specific cell groups show distinctive patterns.
Experienced EL analysts can interpret these patterns and estimate the severity and likely impact on long-term performance.
EL testing in manufacturing
Solar cell and module manufacturers use EL testing as part of production quality control:
Each cell is EL-tested after manufacturing.
Defective cells are sorted out.
Modules are EL-tested before shipment.
EL data is stored in production databases.
Premium manufacturers maintain EL images linked to module serial numbers, supporting future warranty claims and field comparisons.
For Indian solar buyers, premium manufacturer EL imaging is one of several quality indicators alongside flash test data.
Field EL testing
For commissioned solar plants, EL testing diagnoses issues:
Annual EL inspection of utility-scale plants identifies developing defects.
EL testing after lightning, hail, or fire documents damage for insurance and warranty claims.
EL testing during O&M troubleshooting locates failing modules.
EL testing during plant resale validates remaining asset condition.
Field EL testing requires:
Dark environment (night-time or fully covered modules).
Power supply for forward current injection.
Near-infrared camera mounted on a stable platform.
Cable harnesses for connecting to modules.
For large plants, drone-mounted EL cameras at night scan thousands of modules efficiently. The drone captures images while ground equipment injects the current. The combined approach can inspect a 10 MW plant in one or two nights.
EL versus thermal imaging
EL and thermal imaging are complementary diagnostic techniques:
| Aspect | EL Imaging | Thermal Imaging |
|---|---|---|
| Energy source | Electrical (injected current) | Solar (operating cells) |
| Time of day | Night | Day (with sun) |
| Wavelength | Near-IR (1100 nm) | Long-wave IR (8 to 14 microns) |
| Detects | Cell quality variations | Heat anomalies |
| Best for | Microcracks, PID | Hot spots, junction box issues |
| Equipment cost | High | Moderate |
| Field deployment | Drone-based scan, night | Drone-based scan, day |
For comprehensive plant assessment, both are valuable. Many premium O&M programmes include both annual EL and annual thermal imaging.
EL imaging interpretation
Analysing EL images requires experience. Key considerations:
Pattern recognition: Different defect types produce characteristic patterns. Experienced analysts identify these quickly.
Severity assessment: Distinguishes between cosmetic defects (no performance impact) and performance-affecting defects.
Tracking over time: Comparing EL images across years reveals defect progression.
Documentation: Each significant defect should be documented with location (module serial number, position in array), pattern description, and severity estimate.
For warranty claims, EL images alongside IV curve data and field measurements provide strong technical evidence.
Common EL testing mistakes
Treating all dark areas as defects. Some EL darkness is cosmetic; experienced analysts distinguish.
Skipping baseline EL imaging. Without baseline, future EL comparisons lack context.
Performing EL without proper darkness. Ambient light contaminates the imaging.
Using inadequate current. Insufficient injection current produces faint images that miss defects.
Not documenting EL findings properly. Without records, defect progression cannot be tracked.
Mistaking EL for general health check. EL focuses on cell-level defects; other diagnostics catch other issues.
Best practices
For new installations, perform EL imaging at commissioning to establish baseline.
For utility-scale plants, schedule annual EL inspections using drone-mounted equipment.
After significant events (lightning, hail, fire), perform EL imaging to document damage.
For warranty claims, EL imaging is a key technical document.
For O&M, integrate EL findings into the plant’s asset management system.
For lender-grade due diligence, EL data of new modules can be requested as part of incoming inspection.
Standards and references
EL testing methodology is documented in IEC TS 60904-13 (current voltage measurement of bifacial PV devices) and indirectly in IEC 61215 (PV module design qualification). Equipment manufacturers publish detailed application notes.
Related glossary terms
- IV Curve
- Mono PERC
- TOPCon Solar Panel
- HJT Solar Panel
- PID and Anti-PID
- Soiling Loss
- Shading Loss
- Solar Panel Degradation
Key takeaways
Electroluminescence (EL) testing is a non-destructive diagnostic technique that injects forward-bias current into a solar module in the dark, capturing the near-infrared emission with a specialised camera. The resulting image reveals cell-level defects including microcracks, PID damage, hot spots, broken cells, and interconnect failures that are invisible to the naked eye. EL testing is used in cell and module manufacturing, at delivery for incoming inspection, after major events for damage documentation, and as part of annual O&M for utility-scale plants. Drone-mounted EL imaging enables efficient inspection of large plants in a single night.