We clarified the formation mechanism of dendrite crystals from Si melt based on fundamental experiments using a newly developed in-situ observation system. The knowledge was successfully implemented to “dendritic casting method” for realization of Si multicrystals and wafers with controlled grain orientations, grain sizes, and grain boundary characters which have never been achieved before. The dendrite casting method permits to realize Si multicrystals with high-quality and high-uniformity, which is required for development of high-efficiency solar cells to accelerate long-term and global scale installation of solar cells. It has been well known that more than two parallel twins exist at the center of a facetted dendrite. However, the underlying mechanisms have not been fully understood. We proposed a model to involve the formation of a twin boundary at the {111} facet plane, and confirmed its validity. Furthermore, we clarified relationship between the parallel twins and the nucleation of dendrite crystals, and quantified the critical amount of supercooling for the appearance of the dendrite crystals using in-situ observations. Thanks to the fundamental understanding of the growth mechanisms, Si multicrystals with high-quality and high-uniformity have been successfully obtained by the dendritic casting method. Although the growth was performed in a small crucible compared with the industrial scale, the solar cell based on the Si multicrystal-wafers grown by the dendritic casting method exhibited conversion efficiency similar with those based on commercially available high-quality Si multicrystal-wafers, which have been independently confirmed by our industrial partner. This indicates the possibility that further up-scaling of the crucible size results in very high-quality Si multicrystals, which could be comparable with or surpass the quality of Si single crystal. The results were presented at the European Photovoltaic Solar Energy Conference in Milano on 4 September, and were reported in the on-line news of Nikkei Microdevices on 6 September.
Solid/liquid interface of Si during melt growth.
The shape of the interface changed from (a) to (d) with decreasing melt temperature.
Note that a facetted dendrite grew from the facetted interface.
The direction of growth is parallel to the facet planes on the interface.