Effects of germanium doping on the boron-oxygen complex formation in silicon solar cells

The light-induced degradation (LID) due to the boron-oxygen (B-O) complexes is particularly harmful for solar cells made from the boron-doped p-type Czochralski silicon (Cz-Si). Many studies focused on this phenomenon and some processes have been proposed to reduce it. Recently, Cz-Si voluntarily doped with germanium (Ge) was used to inhibit the effect of the B-O complexes activation. We studied here both conventional Cz-Si (CZ) and Ge-doped Cz-Si (GCZ), and compared their sensitivity to the LID. The first result was that solar cells produced with GCZ had the same performances than solar cells made with CZ despite the impurities like iron unintentionally introduced in GCZ by the Ge powder. Then, we found that the degradation under illumination of the efficiency was lower in GCZ only for the solar cells produced from the last solidified part of the ingot (where the Ge content is the highest). These differences were correlated with the amount of interstitial oxygen (O-i) which was found to be lower for the GCZ wafer, possibly due to the formation of Ge-O-i related defects and/or to Ge-enhanced oxygen precipitation effects. At the cell level, we also compared the kinetics of the LID in both materials and confirmed that the degradation was slower in GCZ. Finally, we showed that the mechanism of light-induced regeneration (LIR) also occurs in GCZ solar cells, with similar kinetics with respect to conventional Cz-Si cells. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim