Degradation of carrier lifetime in Cz silicon solar cells

The lifetime degradation induced by light illumination or carrier injection which is observed in Czochralski-grown silicon (Cz-Si) leads to a signific ant decrease of solar cell efficiency. Thus, the reduction of this effect h as a high potential for the improvement of Ct-Si solar cells. In the presen t work both, the analysis of the underlying defect and its technological re duction are discussed. A clear correlation of the ct specific metastable de fect with the oxygen and boron concentration in Ct-Si has been observed. Es pecially, recently performed lifetime measurements on oxygen-free boron-dop ed p-type MCz silicon and gallium-doped oxygen-contaminated Ct-silicon, bot h of which show no degradation, confirm this hypothesis. While the quantita tive correlation between the defect concentration and boron is linear, the increase of the defect concentration induced by the interstitial oxygen con centration is superlinear, i.e. it follows a potential law of power approxi mately 5. Beyond the defect analysis, two different ways to reduce the meta stable defect concentration are discussed. A proper material choice by subs tituting or reducing one of the major components of the metastable defect c an completely avoid the degradation effect. The excellent performance of ox ygen-free MCz-Si and gallium-doped Ct-Si is reflected in the achieved recor d efficiencies of 22.7% and 22.5%, respectively. In standard boron-doped ox ygen-contaminated p-type Ct-Si a strong reduction of the metastable defect concentration can be achieved by a high-temperature process step resulting in an improvement of the stable bulk lifetime by a factor of 2-4. (C) 2001 Elsevier Science B.V. All rights reserved.