Analysis of grain structure evolution via image processing based on optical measurements of mc Si wafers
A fast and thorough characterization of grain structure in multicrystalline silicon (mc-Si) is crucial to improve crystal growth and thus bulk lifetime in solar cells. The presented characterization techniques are based on simple optical measurements on as-cut mc-Si wafers. An insight into the entire brick is gained by connecting 2D-information, computed via advanced pattern recognition techniques, over brick height. We identify robust statistical key parameters. Their development within typical bricks of different cast-Si techniques is compared and it is found that the distinct behavior of different materials in the lower part of the brick subsides towards the brick top where grain size distribution is similar.
Feeding of liquid silicon for high performance multicrystalline silicon with increased ingot height and homogenized resistivity
Abstract Feeding of liquid silicon during the directional solidification process is a promising opportunity for cost reduction by increased throughput and improved material homogeneity due to constant resistivity over ingot height. In this work, a liquid feeding apparatus was developed for an industrial type directional solidification furnace. One n-type G2 sized High Performance multicrystalline ingot with liquid feeding of additional 14 kg of undoped silicon feedstock was crystallized. The resistivity was kept within a range of ±0.1 Ω cm of the target resistivity during the feeding sequence. A smaller mean grain area growth was observed during feeding, whereas the area fraction of recombination active dislocation structures was as low as in a reference ingot. Increased interstitial oxygen and substitutional carbon concentrations were measured for the ingot with liquid feeding. The measured mean bulk lifetime of 190 µs for passivated wafers in the feeding sequence can probably be increased by further pre-melting crucible improvements. For this laboratory experiment, energy reductions of 2% per wafer and time savings of 16% per wafer were realized.
Undercooling and solidification of Si by electromagnetic levitation
Abstract Pure Si droplets were containerlessly undercooled using an electromagnetic levitation method. An undercooling up to 330 K prior to solidification has been reproducibly achieved for bulk samples in size of 10 mm. A transition from faceted growth at lower undercoolings to continuous growth at higher undercoolings was observed through analyses of changes in phase morphologies on the surface of the samples. The transition was caused by existence of a large kinetic undercooling. The nucleation frequency and the crystal/melt interfacial energy are discussed within the frame of Spaepen’s model in terms of the structure of the interface.