Abstract
Wafers from three heights and two different lateral positions (corner and centre) of four industrial multicrystalline silicon ingots were analysed with respect to their grain structure and dislocation density. Three of the ingots were non-seeded and one ingot was seeded. It was found that there is a strong correlation between the ratio of the densities of (coincidence site lattice) CSL grain boundaries and high angle grain boundaries in the bottom of a block and the dislocation cluster density higher in the block. In general, the seeded blocks, both the corner and centre block, have a lower dislocation cluster density than in the non-seeded blocks, which displayed a large variation. The density of the random angle boundaries in the corner blocks of the non-seeded ingots was similar to the density in the seeded ingots, while the density in the centre blocks was lower. However, the density of CSL boundaries was higher in all the non-seeded than in the seeded ingots. It appears that both of these grain boundary densities influence the presence of dislocation clusters, and we propose they act as dislocation sinks and sources, respectively. The ability to generate small grain size material without seeding appears to be correlated to the morphology of the coating, which is generally rougher in the corner positions than in the middle. Furthermore, the density of twins and CSL boundaries depends on the growth mode during initial growth and thus on the degree of supercooling. Controlling both these properties is important in order to be able to successfully produce uniform quality high-performance multicrystalline silicon by the advantageous non-seeding method.
Highlights
Multicrystalline silicon is the dominant material for solar cell production, and its fraction of the total production volume is increasing, accounting for 62.4% in 2017 [1]
A visual impression of the grain size can be obtained from orientation image maps (OIM), or from individual grain images (IGI), where all individual grains are coloured with a unique colour, independent of the crystal orientation
We have argued that the total random angle grain boundary density is a measure of grain size that reflects how crystals with different orientation still pertain to the same origin through growth twinning or twinned dendrites during initial growth
Summary
Multicrystalline silicon is the dominant material for solar cell production, and its fraction of the total production volume is increasing, accounting for 62.4% in 2017 [1]. Its strong position is a consequence of its relatively simple production process compared to the alternatives and its relatively high achievable efficiency. The increasing trend in dominance, despite the fact that multicrystalline silicon cannot be used in the very-high-efficiency cell architectures and demands more balance of system costs, may be a result of its very good process scalability. The material quality has been quite radically improved during the last few years. The material quality that has taken over the market is termed high-performance multicrystalline silicon (HPMC-Si) and has the following characteristics [2,3]:
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