Overview of Cast Multicrystalline Silicon Solar Cells
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MRS BULLETIN/OCTOBER 1993
ficiencies of 17.1%" and 16.4%5 have been obtained for cell areas of 10 cm X 10 cm and 15 cm X 15 cm, respectively. In this report, I will first review the multicrystalline ingot growth method. Next, I will summarize the steps for improving the performance of multicrystalline solar cells. Finally, I will describe some examples of the application of multicrystalline silicon solar cells.
Cast Ingot Growth In 1975, Wacker proposed a new manufacturing method using the Wacker ingot casting process (WICP) for low-cost substrates for terrestrial solar cells, instead of the conventional Czochralski method. WICP uses two different containers for melting and crystallization. The process consists of melting silicon in a quartz crucible, pouring the silicon melt into a preheated mold, and then using directional solidification. The mold is built with graphite elements. The solidification process is carried out in a controlled vertical temperature gradient of the silicon melt in the mold to obtain a block with liners of crystalline silicon from the bottom to the top of the block. Figure la shows a silicon cast block. Contouring and portioning of large-sized ingots from the block is carried out with a band saw technique. Figure lb shows silicon ingots with a cross section of 10 cm X 10 cm, made by cutting the cast block. The silicon ingot is sliced into wafers (or substrates) with a thickness of about 300 /am with methods such as internal diameter (ID) or multiwire (MW) sawing methods. Figure lc shows wafers with areas of 10 cm X 10 cm and 15 cm X 15 cm. Since Wacker proposed the ingot casting process, many kinds of casting methods have been proposed by a large number of research groups, shown in Table I. The casting method is expected to
become cheaper than the conventional Czochralski method. On the other hand, there are disadvantages because the substrates contain stress, impurities, grain boundaries, and so on. Many methods have been examined to make the directional solidification result in columnar crystal growth and to reduce the inner stress, to avoid interaction of molten silicon with the mold and the resulting impurity contamination, and to suppress the generation of cracks. Meanwhile, to reduce the manufacturing cost of the casting ingot, efforts for obtaining a large ingot have been performed and ingots with sizes of 43 X 43 X 28 cm3,6 44 X 44 X 20 cm3,16 and 44 X 44 X 18 cm3,10 with weights of 120 kg, 90 kg, and 80 kg, respectively, have been reported. In 1985, the Solar Energy Research Institute developed a cold-crucible casting technique based on induction heating.17 In this method, neither crucible nor mold is used, and molten silicon is heated and confined electromagnetically by induction power, so impurity contamination from furnace material and consumption of the crucible and mold are avoided. By the vertically pulling continuous casting method, a 25-mm-square cross sectional ingot with a length of 170 mm has been obtained. Crystalox has developed a horizontal zone silicon refining m
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