High-Quality Si Multicrystals with Same Grain Orientation and Large Grain Size by the Newly Developed Dendritic Casting Method for High-Efficiency Solar Cell Applications

Summary

Si multicrystals have many grains with different orientations and sizes, resulting in many grain boundaries with different characteristics. To obtain the high conversion efficiency of the solar cells prepared by Si multicrystals that is very close to that of Si single crystals, Si multicrystals that have grains with a same orientation and proper sizes and have electrically inactive grain boundaries are required. This concept was tried and the dendritic casting method was newly developed to obtain extremely high-quality Si multicrystal ingots and to largely increase the yield of high-quality Si multicrystal ingots. To develop such a new growth technology by understanding the crystal growth mechanism to control the grain orientation and the grain size, the in-situ observation system was newly developed to directly observe the growing interface of Si crystals at temperatures higher than l,400°C. Using the in-situ observation system, it was found that the new growth mode of Si dendrite crystals appeared along the bottom of the crucible at the initial stage of the growth. Such dendrite crystals were very effective to control the grain orientation and the grain size of Si multicrystals. Using the developed dendritic casting method, extremely high-quality Si multicrystal ingots with the same grain orientation and very large grain size were obtained. The conversion efficiency of the solar cells prepared by such Si multicrystals was much higher than that of the solar cells with different grain orientations, especially for the upper parts of the Si ingots.