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1. Overview of EL Testing
Electroluminescence (EL) testing is a critical inspection technique for photovoltaic (PV) modules. By applying a forward bias to solar cells to induce luminescence, it detects defects and performance issues in cells. Traditional EL testing requires a darkroom environment, whereas daytime EL technology overcomes this limitation, enabling effective detection under strong ambient light.
2. Principles of Daytime EL Testing
2.1 Fundamental Working Principle
The core principle of daytime EL aligns with conventional EL, based on the electroluminescence phenomenon in semiconductor materials:
A forward bias is applied to the PV module, injecting minority carriers.
Electrons and holes recombine near the PN junction, emitting photons (typically in the infrared spectrum, ~1150 nm for silicon cells).
2.2 Challenges and Solutions for Daytime Testing
Key challenges from sunlight interference are addressed through:
Optical filtering: Narrowband pass filters (center wavelength ~1150 nm, bandwidth 30–50 nm).
High-current injection: Boosts EL signal intensity.
Synchronous detection: Modulated excitation + lock-in amplification to extract weak signals.
Image processing algorithms: Background subtraction and signal enhancement.
3. Key Technical Components
3.1 Optical Imaging System
High-sensitivity InGaAs infrared camera (spectral range: 400–1700 nm).
Customized optical lens assembly.
Narrowband IR filter (e.g., center wavelength 1150 nm, FWHM 40 nm).
3.2 Electrical Excitation System
High-current power supply (up to 1.2–1.5 × Isc of the module).
Fast-switching control unit (μs-level response).
Four-wire measurement to reduce line losses.
3.3 Synchronization Control Unit
Precision timing controller.
Synchronized camera exposure and current pulses.
Multi-frame averaging for noise reduction.
3.4 Image Processing System
Background illumination correction.
Adaptive contrast enhancement.
Automated defect recognition algorithms.
4. Testing Procedure
Module preparation: Clean surfaces, verify electrical connections.
Parameter setup: Set current (typically 1.1–1.3 × Isc) based on module type.
Pulse excitation: Apply short high-current pulses (50–200 ms).
Image acquisition: Trigger IR camera synchronously.
Image processing: Background subtraction, signal enhancement.
Result analysis: Defect identification and classification.
5. Applications
PV plant inspection: Daytime field testing without module removal.
Production-line QC: 100% inline inspection.
Degradation analysis: Study of PID, LID, etc.
Fault diagnosis: Detection of hotspots, microcracks, soldering defects.
6. Technical Advantages
No darkroom needed: Direct outdoor testing in daylight.
High efficiency: <30 seconds per module.
High resolution: Detects microcracks (<2 mm).
Quantitative analysis: Luminescence intensity measurement (in some systems).
7. Limitations
High current demand: Power supplies must support >10 A.
Material limitations: Less effective for thin-film modules.
8. Conclusion
Daytime EL testing eliminates environmental constraints in PV inspection, offering a fast, efficient tool for plant maintenance and quality control. Advances in optics and image processing are driving the technology toward higher sensitivity, intelligence, and multi-functional integration.
