Multicrystalline silicon crystal assisted by silicon flakes as seeds

- mc-Si ingots were grown by the assistance of silicon wafer fakes as seeds.
- uniform small seeds with ~ 0.5 mm diameter were formed in the interspaces between flakes.
- Epitaxial growth on self-formed seeds effectively reduce the dislocation density.
- The efficiency values of the N-doped solar cells are acceptable.
- Efficiency of Al-BSF cells from the seed assisted mc-Si is well improved.

The seed assisted growth technology has been used to fabricate the high performance multicrystalline silicon (mc-Si) for photovoltaic application nowadays, which can effectively control the dislocation density by forming uniform and small grains. In this work, we have systematically investigated the effect and mechanism of silicon wafer flakes as the seeds for mc-Si growth. Silicon flakes were put on the crucible bottom as nucleation agents by two different settings, i.e., orderly stacking without interspace and intentionally piling up with interspace. It was found that the numerous small and uniform grains with diameter of ~ 0.5 mm were homogenously formed in the intentionally piled up wafer flake seed layer since the liquid silicon drops flew down and solidified in the interspaces during the melting of polysilicon feedstocks. These small self-formed grains served as new seeds, which were better than orderly stacked wafer flakes as seeds. Minority carrier lifetime and photoluminescence mappings have shown that the initial dislocation density of grains with smaller sizes based on the piled silicon flake seeds was much lower than those without seeds or stacked silicon flake seeds, and meanwhile the dislocation propagation was suppressed by means of smaller grain sizes due to the denser grain boundaries. As a result, the corresponding average conversion efficiency of Al-BSF processed solar cells from the mc-Si assisted by piled silicon flakes as seeds was absolutely 0.29% higher than those without seeds and 0.17% higher to those assisted by stacked silicon flakes.