Nanometer-Scale Carrier Imaging of Potential-Induced Degradation in c-Si Solar Cells

We report on nm-resolution imaging of chargecarrier distribution around local potential-induced degradation (PID) defects using scanning capacitance microscopy. We imaged cross sections of heavily field-degraded module areas as cored out and selected by mm-scale photoluminescence imaging. Localized areas with abnormal carrier behavior or junction damage were found: the apparent n-type carrier extends vertically into the absorber to ~1-2 um from the cell surface, and laterally in similar lengths; in defect-free areas, the n-type carrier extends ~0.5 um, which is consistent with the junction depth. For comparison, we also investigated areas of the same module exhibiting less PID stress, and did not find any such heavily damaged junction area. Instead, we found slightly abnormal carrier behavior, where the carrier-type inversion in the absorber did not occur, but the p-type carrier concentration changed slightly in a much smaller lateral length of ~300 nm. These nanoelectrical findings suggest that the existing extended defects, which may not be significantly harmful to cell performance, were changed by PID to heavily damaged junction areas.