High-Resolution Lock-in Thermography Investigation on Industrial Multicrystalline Silicon Solar Cells

Industrial multicrystalline silicon (mc-Si) solar cells with different types of shunts have been analyzed in detail by dark lock-in thermography (DLIT). Several types of nonlinear shunts were found in our samples and most of them could only be detected in low forward-bias images of DLIT. However, we also observed nonlinear shunts that are only visible or have much stronger signal under the low reverse-bias than equivalent forward-bias condition, which is a new finding compared with the common observations. The edge of an mc-Si solar cell was found vulnerable for shunting. A weak leakage current around edges was frequently observed under 0.5 V forward bias on both shunted and normal cells. It reveals that the edge is one of the major recombination paths under the cell operation condition, which is due to the imperfect edge passivation and material quality limitation of mc-Si. Light-beam-induced current (LBIC) was also applied on one material-induced shunt. LBIC mapping with long wavelength revealed the degraded current response due to poor wafer quality. However, an LBIC image of short wavelength did not show the defect structure because the current was dominated by the Auger recombination, while not influenced by the bulk lifetime. Some pre-breakdown sites were found in the material-induced shunt sample and were only visible under the reverse-bias condition of DLIT.