Improvement of the Production Yield of Spherical Si by Optimization of the Seeding Technique in the Dropping Method

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Spherical Si with a diameter distribution of approximately 1.0 mm, which is applicable to spherical solar cells, was formed by the dropping method. To control the solidification of the molten Si droplet to occur at shallow undercooling, a seeding technique was developed. In this paper, we describe the optimization of the seeding technique with emphases on high production yield and high minority carrier lifetime. An effective seeding supply system was first introduced. It was found that the seeding with Si powder causes impurities, such as Fe, into Si spheres, and the crystallinity becomes poor with increasing yield. SiO, SiO2, Si3N4, and boron nitride powders were shown to have the same seeding effect as Si powder. Among these, SiO2 was the best candidate. The seeding with SiO2 powder did not increase the concentration of Fe and O impurities and did not decrease the crystallinity. The dependences of the yield on the size and the density of SiO2 powder were systemically investigated. The best yield of nearly 70% was realized at a high SiO2 powder density.
2007-09-15