Multicrystalline silicon material: Effects of classical and rapid thermal processes
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Multicrystalline silicon material: Effects of classical and rapid thermal processes J. C. Muller CNRS, Laboratoire de Physique et Applications des Semiconductors (UPR292), BP 20, 67037 Strasbourg Cedex 2, France
S. Martinuzzi Laboratoire de Photo´electricit´e des Semiconducteurs (LPSC), Universit´e Aix-Marseille, 13397 Marseille Cedex 13, France (Received 2 March 1998; accepted 2 March 1998)
For photovoltaic applications silicon is still the predominant material. Besides monocrystalline Czochralski wafers (Cz-Si), multicrystalline sheets (mc-Si) play an important role in terrestrial power applications (almost 50%). Large mc-Si ingots (up to 250 kg) are now produced in large scale by the industry using various directional solidification methods in appropriate crucibles (or molds). However, if the crystallographic properties are now quite satisfactory (columnar structure with large grains of more than 1 cm2 , dislocations and intragrains defects), multicrystalline silicon contains larger quantities of impurities than single crystalline silicon which can have detrimental effects on the bulk minority carrier diffusion length (Ln,p ). These impurities, including metals as well as high concentrations of carbon and/or oxygen, can degrade the photovoltaic properties of solar cells. Thermal treatments such as gettering, performed in a classical or rapid thermal furnace, studied separately or in conjunction with the doping steps can limit or avoid the degradation of the bulk diffusion length, but its efficiency is strongly dependent on the presence of these impurities in Si.
I. INTRODUCTION
Photovolta¨ıc (PV) cells, which convert directly the solar energy into electricity, are probably the most effective alternate energy source to conventional power supplies. The world demand for terrestrial applications of photovoltaic systems has increased first for isolated areas and now in connection with grids. Many kinds of solar cells have been developed, based on various semiconductor materials, including compounds. Although silicon, which was the first, is still probably the best candidate for future large scale application due to the great abundance, technical advance, and environmental savety of this material. Among them, solar cells from cast multicrystalline silicon (now referred as mc-Si) and in the past also as cast polycrystalline or semicrystalline silicon can be considered to be the most promising type of material capable to achieve low cost high rate production without much efficiency lost as compared to single crystals (Sc-Si). Multicrystalline silicon substrates contain many kinds of defects and residual impurities as compared to single-crystal silicon so that the efficiency was largely inferior to that of conventional Czochralski (Cz) or float-zone (FZ) silicon. However, recent research on multicrystalline silicon, including progress on the knowledge of the material and of this thermal reaction during the cell processing, has J. Mater. Res., Vol. 13, No. 10, Oct 1998
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