Increasing cell efficiency of S-web silicon ribbon material using synthetic silica crucibles

For solar cell application Si ribbons 12 cm wide and up to 150 m long have been grown with a pulling speed of 1m/min by the S-web technique. The ribbon material is polycrystalline with grain sizes in the order of mm2. This should allow to process cells with efficiencies up to 15%. Efficiencies of 12% for test solar cells of 20mm x 20mm in size have been reported earlier. In order to find out if impurities are the real limiting factor for higher efficiencies, a neutron activation analysis was carried out on Si ribbon material, carbon fiber net, silica crucible, and Si starting material, respectively. The silica crucible was identified as the source for Zr, Mo, W impurities. In the case of Fe a clear statement as to its origin was not possible. Replacing most of the natural silica by synthetic silica as crucible material eliminated some of the impurities. Material grown from this crucible resulted in a maximum cell efficiency of 13%. In order to achieve efficiencies exceeding 13% it is n ecessary to either completely replace natural by synthetic silica crucibles and/or to keep the time of contact between melt and crucible low.