Search

We studied the correlation between the appearance of defects and of breakdowns in fully processed solar cells by electroluminescence imaging. Images under forward and reverse bias were recorded, revealing defective areas and breakdown sites of the cells, respectively. In the experiments presented here an InGaAs camera system was used, which allows us to detect short wave infrared light in the 900–1700 nm range. Applying a forward bias the band-to-band luminescence (1.1 eV/1150 nm) and the defect-related luminescence (0.8 eV/1550 nm) were imaged. Both imaging techniques reveal mainly dislocation-rich areas and grain boundaries. Applying a reverse bias (approximately −10 V), breakdown luminescence occurs. It was found that the distribution of the breakdown sites across the solar cells is different from the distribution of the defects showing luminescence at 0.8 eV (grain boundaries, dislocations). The spatial separation between those defects and the breakdown sites was clearly evidenced by high resolution imaging. Possible reasons for such spatial separation are discussed.

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.