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

The efficiencies of solar cells based on multicrystalline silicon (mc-Si) have reached 17% even employing high-throughput crystallization steps and industrial-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 boundaries, crucially depends on the crystallization conditions, while, with regard to impurities, electrically active transition metals, such as iron, are well-known to seriously reduce the minority carrier lifetime. A similarly important role, however, is played by oxygen. Various oxygen or oxygen-containing defect centers showing strong recombination activity may form in monocrystalline silicon as well as in mc-Si. In mc-Si blocks the formation of so-called thermal donors and nitrogen-oxygen complexes can take place during the relatively slow cooling of the ingots. Thermal donors and nitrogen-oxygen complexes lead to reduced lifetimes especially in the edge regions of the ingot. Whereas this lifetime reduction is hardly efficiency-relevant as long as annealing steps above 600°C for several minutes are implemented in solar cell processing, another species of oxygen donor, the new donor, forms in the temperature range between 600 and 900°C that is frequently used for solar cell fabrication. For silicon with a high oxygen content such as the Bayer RGS (ribbon growth on substrate) material, the new donors seem to be the most efficiency-relevant defects which can only be prevented using well-adjusted temperature profiles during crystallization and solar cell processing. Whereas monocrystalline silicon can benefit from high oxygen content through internal gettering steps in microelectronic device processing, a substantial improvement of mc-Si for solar cells is achievable by lowering the oxygen content. Oxygen contents considerably below those of monocrystalline silicon are therefore state of the art for modern high-throughput production material fabricated by the block-casting technology.