Distribution of Defects and Breakdown Sites in UMG-Si Solar Cells Studied by Luminescence Imaging

Electroluminescence imaging under forward (EL) and reverse bias (ReBEL) has been applied to correlate defect distribution and breakdown sites in solar cells made of UMG-Si. An InGaAs camera system is used for luminescence detection, providing access to band-to-band and to the sub-band gap radiative transition in silicon. In contrast to previous papers the ReBEL reported here is observed only in the near-infrared (1.25 eV - 0.6 eV) spectral range and no visible luminescence has been detected. It is found that ReBEL intensity correlates with dark I-V characteristics. The overall distribution of breakdown sites in cells is different from the distribution of dislocations exhibiting luminescence at 0.8 eV (grain boundaries, defects). For the first time, a spatial separation between such defects and breakdown sites is clearly established by high resolution images, giving evidence that the defects showing sub-band gap luminescence do not take part in breakdown. It is further concluded that such radiative defects reduce the affinity for breakdown in their vicinity.