Principles of PV Module EL Testing: The "X-ray" for Revealing Internal Defects‌

In the manufacturing and quality control process of photovoltaic (PV) modules, the EL (Electroluminescence) defect inspection instrument plays a critical role, akin to performing a precise "X-ray" scan on the modules. Based on the principle of electroluminescence (EL), it non-destructively and with high sensitivity reveals various internal defects invisible to the naked eye, making it a key piece of equipment for ensuring long-term reliability and power generation performance of PV modules.

The Physical Basis of Electroluminescence (EL)

The EL phenomenon is essentially an inverse process of the photovoltaic effect. When a PV module is subjected to a forward DC bias (i.e., the P-region connected to the positive electrode and the N-region to the negative electrode) under the control of the EL defect inspection instrument, the injected current forces the PN junction into a forward-biased state. At this point, the injected minority carriers (electrons and holes) meet during diffusion and undergo radiative recombination. In silicon materials, this recombination releases photons with wavelengths in the near-infrared (NIR) range (approximately 1100–1200 nm). In simple terms, normal, high-performance cell areas emit light of a specific wavelength when driven by current.

The core function of the EL defect inspection instrument is to capture and image this faint near-infrared light emitted by the module itself, thereby converting the internal current distribution and cell integrity into a visual representation. The luminescence intensity is directly related to the local current density and minority carrier lifetime. Thus, areas with stronger luminescence indicate better electrical performance, while areas with weak or no luminescence suggest defects or performance degradation.

Core Components and Workflow of the EL Defect Inspection Instrument

A typical EL defect inspection instrument consists of several key systems working in synergy:

1. ‌Precision-Controlled DC Power Supply System‌: Provides stable, precisely adjustable forward bias current to the module under test. The current magnitude is typically set based on module specifications (e.g., number of cells, cell type) to ensure sufficient EL signal intensity while avoiding excessive current that could cause damage or hot spots. The power supply must be highly stable and controllable.

2. ‌High-Sensitivity Near-Infrared Imaging System‌: This is the "eye" of the EL defect inspection instrument. The core component is a specially designed NIR camera (usually equipped with high-quantum-efficiency Si-CCD or InGaAs sensors) that is highly sensitive to photons in the 1100–1200 nm wavelength range. The camera is typically fitted with a large-aperture, low-distortion lens and mounted on an adjustable-height stand. To obtain clear images, the entire testing process must be conducted in complete darkness to eliminate any external stray light interference.

3. ‌Image Acquisition and Processing System‌: The control software of the EL defect inspection instrument drives the power supply, controls the camera exposure time (typically ranging from hundreds of milliseconds to several seconds), and triggers image capture. The acquired raw EL images undergo noise reduction, enhancement, contrast adjustment, and other algorithmic processes to optimize defect visualization. Advanced EL defect inspection instruments also feature automatic image stitching (for large modules) and intelligent defect recognition and classification.

4. ‌Module Support and Positioning System‌: Provides a stable platform for placing the module and may include movement or rotation mechanisms to facilitate sectional imaging or angle adjustment for large modules. Some EL defect inspection instruments are integrated into production lines with automated transport and positioning capabilities.

Major Defect Types Revealed by EL Imaging

The images generated by the EL defect inspection instrument serve as a detailed "health map," where different grayscale or color levels represent varying luminescence intensities, thereby mapping multiple internal defects:

• ‌Cracks/Microcracks‌: Appear as thin, sharp black lines or irregular dark areas. Cracks block the lateral current path within the cell, resulting in no or weak luminescence in the affected regions. The EL defect inspection instrument is the most effective tool for detecting microcracks (invisible to the naked eye).

• ‌Broken Fingers/Poor Soldering‌: Broken busbars or finger lines appear as thin black lines, while poor soldering between cells and interconnects (ribbons) manifests as localized or continuous dark bands/spots under the ribbons. These defects impede current collection. The EL defect inspection instrument can precisely locate such connection failures.

• ‌Cell Fragmentation‌: Severe physical damage, appearing as large, irregular black areas.

• ‌Dark Cores/Spots‌: Circular or irregular dark spots in the center or localized areas of the cell. These may be caused by material impurities, uneven diffusion, sintering issues, or localized PID (potential-induced degradation), leading to increased recombination centers and reduced minority carrier lifetime.

• ‌Low-Efficiency Cells/Mismatch‌: Entire cells or localized areas exhibit significantly weaker luminescence than surrounding normal cells. This may result from material quality variations, process fluctuations (e.g., diffusion, passivation), or minor microcracks.

• ‌Shunt Defects‌: Include:

  • ‌Edge Shunts‌: Bright lines along cell edges, indicating current leakage paths where current is concentrated.
  • ‌Internal Shunts/Pinholes‌: Small bright spots within the cell, typically localized PN junction short circuits where current bypasses, causing abnormal brightness in the spot and potential darkening of surrounding areas.

• ‌Firing Defects‌: Affect the contact between metal electrodes and silicon, potentially causing poor local contact (dark areas) or over-firing (possibly forming bright leakage points).

• ‌Process Contamination‌: Introduced impurities may enhance localized recombination, forming dark areas with specific patterns in the image.

Key Elements of Standardized EL Testing

To ensure repeatability, comparability, and accuracy of EL defect inspection results, the following factors must be strictly controlled:

• ‌Darkroom Environment‌: Testing must be conducted in a completely light-sealed space. Any external light source can interfere with the weak EL signal, reducing image signal-to-noise ratio and contrast.

• ‌Current Injection Settings‌: The applied current is typically a percentage (e.g., 75%–100%) of the module's nominal short-circuit current (Isc) or set according to standards (e.g., IEC TS 60904-13). Too low a current results in weak signals and difficulty identifying defects, while too high a current may cause thermal effects or even module damage. The EL defect inspection instrument must precisely set and stably output the required current.

• ‌Exposure Time Optimization‌: Camera exposure time should be dynamically adjusted based on module type, injected current, and camera sensitivity. Too short an exposure results in dark, noisy images, while too long an exposure may increase thermal noise, reduce efficiency, or cause saturation. The EL defect inspection instrument software typically includes automatic or semi-automatic exposure functions.

• ‌Module Temperature‌: Higher temperatures increase non-radiative recombination, reducing EL intensity. The module temperature should remain stable during testing (ideally near room temperature), especially avoiding significant heating due to injected current. Fast-imaging EL defect inspection instruments help minimize temperature effects.

• ‌Image Analysis and Criteria‌: EL images must be evaluated against clear, standardized (e.g., IEC 63202-1, IEC TS 60904-13) or internal defect classification criteria to distinguish acceptable defects from rejectable ones. The EL defect inspection instrument software should support defect annotation, measurement, and report generation.

As an indispensable non-destructive testing tool in the PV industry chain, the EL defect inspection instrument, with its electroluminescence-based imaging technology, provides manufacturers with a unique perspective into the internal "health" of modules. It not only effectively identifies critical or potentially harmful defects such as cracks, broken fingers, and poor soldering—ensuring strict quality control—but also aids in process diagnosis, failure analysis, and long-term reliability studies. With continuous advancements in speed, resolution, and intelligent analysis, the EL defect inspection instrument will continue to provide robust technical support for improving the quality, performance, and customer confidence in PV modules.