On the degradation of Cz-silicon solar cells

The present work aims at the analysis and technological reduction of the metastable defect responsible for the degradation of efficiencies of solar cells fabricated from Cz-silicon under illumination or forward bias. Lifetime measurements on Cz-silicon wafers show a clear correlation between the oxygen and boron content and the defect concentration. Additionally, oxygen-doped FZ-wafers have been analyzed: While phosphorus- and oxygen-doped FZ-silicon and oxygen-free boron-doped FZ-silicon show no degradation effect, boron- and oxygen-doped FZ-silicon exhibits the same metastable defect behavior as boron-doped Cz-silicon in the absence of other contaminations. This is in coincidence with the recent suggestion that BiOi could be the responsible defect. We demonstrate that the defect concentration can be reduced significantly by high-temperature processes. Either a thermal oxidation or a solar cell process, both optimized for Cz-silicon at Fraunhofer ISE, increase the stable lifetime after degradation by a factor of 2 to 3.