Karl-Heinz Eisenrith

New methods to prepare high-purity silica

Two methods are described to prepare high-purity silica to be employed as a low-cost starting material for solar-grade silicon production. In the first process, abundantly available quartz sand is purified by fusing it with glass forming oxides to form a melt from which thin glass fibres are drawn. Subsequent treatment of the fibres with hot HCl leads to an ion exchange whereby all non-siliceous oxides are removed from the glass network leaving an insoluble matrix of high-purity silica. The second method employs sodium hexafluosilicate (Na2SiF6), prepared from fluosilic acid, a by-product of the fertilizer industry. The Na2SiF6 is thermally decomposed to yield gaseous SiF4 which is subsequently hydrolysed in an ammonia solution. With both methods silica having a purity of >99.99% was obtained.

Production of Solar Grade Silicon in an Arc Furnace Using High Purity Starting Materials

The carbothermic reduction of silica has been adopted to produce solar-grade silicon using high-purity raw materials. Inexpensive and abundant quartz sand is purified by fusing it with glass-forming oxides to form a melt from which glass fibers are drawn. Subsequent treatment of the fibers with hot HC1 leaches out all impurities, resulting in high-purity Si02 analysed to have B, P and transition metal concentrations of less than 1 ppmw. High-purity carbon (B, P and transition metal concentration < 1 ppm) is prepared by treating carbon black with hot HC1. When reacting these purified materials in a small arc furnace, the impurity concentration of the silicon obtained corresponded to the impurities present in the starting materials. A three-phase, 550 kVA-arc furnace was constructed to prepare silicon on a larger scale. Quartz and charcoal were used as raw materials to gain experience in operating the furnace and to study the process parameters. The silicon produced from these impure materials was further purified employing the Czochralski method.

On-line manufacturing of Si solar cells compatible with high speed Si-ribbon growth

For the terrestrial photovoltaic application in the gigawatt range new on-line processing steps are necessary. Applying a new production process for the pn-junction large area (10 cm x 10 cm) mono-crystalline solar cells have achieved conversion efficiencies of 13% and polycrystalline S-Web solar cells efficiencies of more than 10%. With a new grid contact preparation technique solar cells of monocrystalline wafers achieved 12,4% efficiency and S-Web solar cells achieved up to 8%. Both processes can be done in a conventional production line and include a reasonable potential of improving the solar cell quality and cost. The methods and the results are discussed in detail.

Removal of Carbon and SiC from Silicon Prepared by Advanced Carbothermic Reduction

High-purity silica and carbon were employed as starting materials to produce solar-grade silicon by carbothermic reduction. To remove carbon and SiC-particles from the silicon melt, the p-type material was subjected to directional solidification. The resulting silicon has an average carbon concentration of about 5 ppmw and can be either converted into single-crystal ingots by one Cz-pull or cast into polycrystalline ingots. Solar cells prepared from both types of material have efficiencies of up to 13.4% (AM1).