The Judgment Criteria for Hidden Cracks in Photovoltaic Cell EL Testing

In the production and quality control of photovoltaic (PV) cells, the PV cell electroluminescence (EL) tester plays a critical role. It operates on the principle of electroluminescence (EL), applying a forward current to the cell to induce near-infrared light emission from within. A high-sensitivity camera captures these light signals, generating a visual image. On a flawless cell, the EL image should exhibit uniform brightness. However, areas with crystalline structure defects or mechanical damage (i.e., microcracks) show reduced or no luminescence due to enhanced carrier recombination, appearing as distinct dark lines, spots, or specific patterns in the image. Accurately identifying these anomalies and assessing their severity based on scientific criteria is essential for ensuring the long-term reliability of PV modules.

The microcracks revealed by the PV cell EL tester vary in form, and their evaluation standards primarily focus on the following key dimensions:

1. ‌Crack Length and Continuity‌:

   • ‌Key Consideration‌: Crack length is the primary factor. Longer cracks (especially those spanning or nearly spanning the entire cell) pose greater risks than shorter microcracks. Continuous long cracks are more prone to propagation under stress during subsequent manufacturing processes (e.g., lamination, installation) or long-term operation, eventually leading to complete cell failure.
   • ‌Standard Focus‌: Industry norms typically set thresholds for total crack length, with cracks exceeding specific lengths classified as critical defects. A single long crack is generally more hazardous than multiple shorter cracks combined.

2. ‌Crack Location and Orientation‌:

   • ‌Key Consideration‌: The physical position of the crack is crucial.
     • Busbar Area: Cracks on or adjacent to busbars carry the highest risk. Busbars are the primary current collection channels; cracks here can directly disrupt current flow, causing significant power loss or total cell failure.
     • Finger Grid Area: Cracks intersecting or parallel to busbars are less severe but may still impair local current collection. Perpendicular cracks pose relatively lower risks but can cause localized power loss.
     • Cell Edge: Edge cracks, especially those extending to the edge, face higher stress during encapsulation and operation, increasing the likelihood of propagation, cell breakage, or fragmentation.
   • ‌Standard Focus‌: Stricter criteria apply to cracks near busbars, those intersecting critical current paths, or located at edges.

3. ‌Crack Morphology‌:

   • ‌Key Consideration‌: The shape and pattern of cracks influence their severity.
     • Linear Cracks: Typically caused by mechanical stress, with clear paths.
     • Dendritic/Radial Cracks: Often result from point-impact damage, radiating outward and affecting larger areas.
     • Intersecting Cracks: Multiple cracks forming a grid, severely dividing the active cell area and causing large-scale failure.
     • Parallel Crack Clusters: Dense parallel cracks significantly weaken mechanical integrity in the affected region.
   • ‌Standard Focus‌: Intersecting, dense dendritic, or widespread parallel cracks are usually deemed critical. Isolated linear cracks are evaluated based on length and location.

4. ‌Impact on Electrical Integrity‌:

   • ‌Key Consideration‌: The core criterion is whether the crack disrupts current flow, creating electrically isolated "islands" (areas where generated current cannot reach the busbar).
   • ‌Standard Focus‌: Any crack causing clear electrical isolation (e.g., complete busbar disconnection or closed-loop isolation) is classified as a critical defect, rendering the cell non-functional.

‌Typical PV Cell EL Tester Evaluation Workflow‌:

1. ‌Image Acquisition‌: Obtain clear EL images under standardized current, exposure time, and other parameters.
2. ‌Anomaly Detection‌: Identify dark lines, spots, or other irregularities manually or via AI-powered image analysis.
3. ‌Feature Analysis‌: Assess detected microcracks for length, location (relative to busbars/edges), morphology (linear, dendritic, intersecting), and continuity.
4. ‌Standard Comparison‌: Match analysis results against predefined written criteria, which may include:
   • Industry Standards: Guidelines like IEC TS 62941.
   • Manufacturer-Specific Standards: Stricter internal thresholds based on product design, process capabilities, and quality goals (e.g., allowable crack length, location maps, or acceptable crack types).
5. ‌Grading‌: Classify cells as:
   • Pass (no cracks or within acceptable limits),
   • Downgrade (defects permitting limited use), or
   • Reject (critical defects).

‌The Importance of Unified Standards‌:
In the widespread application of PV cell EL testers, establishing and rigorously enforcing clear, consistent, and scientific microcrack evaluation standards is foundational for module quality and long-term plant performance. These standards serve as the core reference for production quality control and provide precise feedback for process improvement. By accurately identifying and eliminating cells with hazardous microcracks, manufacturers can prevent potential failure points in modules, significantly reducing risks of power degradation or hot-spot failures during transportation, installation, and operation. This ultimately ensures stable PV plant performance and return on investment.