Analysis on Magnetic Images of Solar Cell by Laser-SQUID Microscope

We have developed a laser-superconducting quantum interference device (laser-SQUID) microscope based on a scanning SQUID probe microscope with a laser excitation system. This microscope can image the local magnetic field distributions detected by the SQUID above a semiconducting sample, which is caused by the photocurrent from the laser excitation spot. In this study, we analyzed the laser-SQUID microscope images of a polycrystalline silicon solar cell equipped with comb-shaped top electrodes. Moreover, these images were obtained by using not only a sample-scanning mode but also a probe-scanning mode. In the sample-scanning mode, the images correspond to the changes of the magnetic field intensity against the positional changes of the laser spot over the sample. On the other hand, in the probe-scanning mode, the images correspond to the magnetic field distribution around a laser spot excited at a fixed position on the sample. Those results indicated that the photocurrent mainly reached the electrode neighbor to the laser spot, and diffused from the electrode to all the other electrodes. Due to the current flow along the electrodes, the image taken by the sample-scanning mode showed the gradually inclined background magnetic signal.