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.
Formation mechanism of parallel twins related to Si-facetted dendrite growth
We propose a simple model to explain formation mechanism of parallel twins that are related to Si-facetted dendrite growth, and attempt to confirm its validity by in situ observations. The model involves the formation of a twin boundary at the {1 1 1} facet plane on the growth interface followed by formation of another twin parallel to the first one. Subsequently, a facetted dendrite grows parallel to the facet plane in this model. This growth behavior was successfully observed by growth experiments.
Grain control in directional solidification of photovoltaic silicon
Abstract Directional solidification (DS) has become the major process for growing multi-crystalline silicon (mc-Si) for solar cells in the photovoltaic industry. The control of grains, as well as the grain boundaries, is particularly important to the crystal quality, and thus the solar cell efficiency. In this paper, we review the progress in the grain control of DS mc-Si from lab-scale to industrial-scale experiments. The control of the growth front was found effective in improving the grain size, but the grain size was found decreased with growth due to the sub-grain formation. With a better control of nucleation and grain competition by increasing the undercooling through enhanced uniform or spot cooling, grains with more Σ3 or twin boundaries were obtained. As the grain size increased with height, the growth of dislocations was found much slower than that without grain growth. The conversion efficiency of the solar cells fabricated from the wafers with grain control was significantly improved. Moreover, the seeded growth was also discussed.