Laser SQUID Microscope for the Evaluation of Solar Cell

A laser SQUID microscope is one of the semiconductor inspection tools. The main feature of this microscope is noncontact detection of the defects. In the previous work, we showed that it could clearly reveal the electrically inactive grain boundaries [1]. In this work, for the further understanding of the imaging mechanism, we investigated a piece of a polycrystalline silicon solar cell as a model sample, which was cut without including the top electrodes. In the magnetic images taken by the laser SQUID microscope, the contrast change was recognized at the grain boundaries similarly to the previous results. The detected distributions of the magnetic field over the whole sample were drastically changed depending on the relative position between the needle probe and the laser spot. We also scanned the needle over the sample with the laser spot fixed on a certain sample position by optical fiber in order to estimate the photocurrent path from the laser spot. These magnetic images imply that the photocurrent induced by laser irradiation did not flow simply. We found that the laser SQUID microscope images were affected by the complicated factors.