Eddy current O-scan and C-scan imaging techniques for macrocrack detection in silicon solar cells

Crack detection is important in the quality control of silicon-based solar cells (SSC). Silicon wafers are semi-conductive and cracks disturb the local conductivity and even induce anisotropy in conductivity of SSC. Eddy current method, which can detect the change of conductivity in the material, is a potential tool for detecting cracks in SSC. In this study, an experimental set-up is proposed to conduct both eddy current O-scan and C-scan imaging to the SSC. The eddy current O-scan was firstly used to investigate the anisotropy in the conductivity of SSC. Through comparing the inherent and crack-induced anisotropy in impedance (amplitude ratio and phase lag), macrocracks could be detected. The changes in the measured directional impedance could roughly reflect the shape of the cross-type macrocrack. In the C-scan mode, multiple slit defects were prepared to simulate macrocracks. Though the lateral resolution of the sensor was limited, the crack detection ability of the sensor could be evaluated with the C-scan imaging results. A slit defect with a width of 0.5 mm and a length of 1 mm was detected. A new criteria, the distance between feature zero-crossing points in the first derivative of the phase lag curve accurately reflected the actual length of the slit.