Formation and annihilation of oxygen donors in multicrystalline silicon for solar cells

The efficiencies of solar cells based on multicrystalline silicon (mc-Si) h ave reached 17% even employing high-throughput crystallization steps and in dustrial-relevant solar cell processes. The efficiency of multicrystalline solar cells is governed by crystal defects, impurities and the interaction of both. The number of crystal defects, such as dislocations and grain boun daries, crucially depends on the crystallization conditions, while, with re gard to impurities, electrically active transition metals, such as iron, ar e well-known to seriously reduce the minority carrier lifetime. A similarly important role, however, is played by oxygen. Various oxygen or oxygen-con taining defect centers showing strong recombination activity may form in mo nocrystalline silicon as well as in me-Si. In me-Si blocks the formation of so-called thermal donors and nitrogen-oxygen complexes can take place duri ng the relatively slow cooling of the ingots. Thermal donors and nitrogen-o xygen complexes lead to reduced lifetimes especially in the edge regions of the ingot. Whereas this lifetime reduction is hardly efficiency-relevant a s long as annealing steps above 600 degrees C for several minutes are imple mented in solar cell processing, another species of oxygen donor, the new d onor, forms in the temperature range between 600 and 900 degrees C that is frequently used for solar cell fabrication. For silicon with a high oxygen content such as the Payer RGS (ribbon growth on substrate) material, the ne w donors seem to be the most efficiency-relevant defects which can only be prevented using well-adjusted temperature profiles during crystallization a nd solar cell processing. Whereas monocrystalline silicon can benefit from high oxygen content through internal gettering steps in microelectronic dev ice processing, a substantial improvement of me-Si for solar cells is achie vable by lowering the oxygen content. Oxygen contents considerably below th ose of monocrystalline silicon are therefore state of the art for modern hi gh-throughput production material fabricated by the block-casting technolog y. (C) 2000 Published by Elsevier Science S.A. All rights reserved.