Pinhole detection in Si solar cells using Resonance Ultrasonic Vibrations

Resonance Ultrasonic Vibrations (RUV) methodology was developed to accurately and automatically detect mechanically unstable wafers and cells with millimeter-length cracks. It was observed by cell producers that other mechanical problems possess a high probability of wafer/cell breakage in production. A small, sub-millimeter diameter “pinhole” represents a seed point defect which dramatically reduces the cell strength and ultimately leads to breakage and yield reduction. To address this production problem, we designed a new cell inspection protocol to identify the wafers and cells with pinholes using an addition to the RUV system called the Activation Station. Pinholes were introduced by indentation in a commercial grade single- or multi-crystalline Silicon solar cells and revealed by a high resolution Scanning Acoustic Microscopy (SAM) and in further details by Scanning Electron Microscopy combined with Focused Ion Beam with cross section capability. It was established that after the indentation, a pinhole gives rise to sub-millimeter length seed cracks which elongate in the form of cross-cracks in Cz-Si cells, and lead to cell fracture. To observe and screen out cells with this type of flaws the RUV system was upgraded with the Activation Station. A new RUV-AS cell inspection protocol was realized. Our tests show that pinholes located at the central area of the cell are covered with 100% accuracy.