Developments in silicon sheet technology

Polycrystalline sheets (standard size: 100 mm x 200 mm) for solar cells are grown from silicon powder layers by a zone-melting process using incoherent light as heat source. As revealed by metallographic methods, grains extend to some cm in length, some mm in width and over the whole sheet thickness. Texture analysis from Laue diffraction patterns yielded a predominance of (211) in growth direction. The surface orientation came out to be random. Variations in etch-pit-densities from 10 high 3/qcm to 10 high 6/qcm were correlated significantly with variations in EBIC contrast. Solar cells of 4 qcm showed efficiencies up to 12.1% after a dramatic increase of up to 40% due to hydrogen implantation. EBIC analysis showed this improvement to be attributed to intra-grain-defect passivation rather than grain boundary passivation