Nucleation and Early Growth of Multicrystalline Solar Cell Silicon
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As the production of solar cell silicon evolves and new concepts are introduced, the need for a more complete understanding of the basic principles of silicon solidification is also increasing. Nucleation has previously been studied under near homogeneous conditions in elevated drop experiments, but little is known about the nucleation mechanisms and initial growth of silicon in silicon nitride coated crucibles. The motivation behind this study was to learn more about which nucleation mechanisms are dominant during the solidification of multicrystalline silicon, and which factors are controlling grain size and orientation. The work was divided into four separate experimental stages: • Wetting experiments • Nucleation experiments on droplets • In-situ experiments • Small scale directional solidification experiments The first part of the study investigated wetting behavior of liquid silicon on silicon nitride coated silica with variations in oxygen concentration of the coating. It was shown that the wetting angle increased with increasing oxygen concentration. The nucleation undercooling of silicon droplets was studied as a function of substrate coating roughness, thickness, oxygen concentration and substrate material in a sessile drop wetting furnace. The results showed that the undercooling for silicon was not dependent on any of the variations in coating properties, only by changes in substrate material. In-situ experiments were performed to study the growth rate and grain size as a function of cooling rate (10 and 30 K/min), variation in coating structure, and silicon feedstock grade. Two grades of silicon were used, a pure electronic grade and a compensated silicon with a carbon concentration of 19 ppmw. The growth rate and grain size were only affected by changes in cooling rate. A high cooling rate increased both growth rate and the number of grains nucleating. The coating structure and impurity variation did not alter neither growth rate nor grain size. A last set of experiments was performed in a small scale Bridgman solidification furnace. Solidification rate and silicon grade were investigated. Ingots were solidified with two different rates, and pulled out of the hot zone with the speed of 0.2 and 50 mm/min. The nucleation temperature was measured for the fastest solidified ingots. It was shown that the increased carbon content in compensated silicon, as compared to polysilicon, does not affect the grain size or orientation in the bottom of the ingot. It was also shown that the slowly solidified ingots had a similar grain size to the dendrite-like microstructure in the bottom of the fast solidified ingot. This indicates that it is the initial growth process, rather than nucleation, that needs to be controlled in order to obtain larger grains in the multicrystalline ingot. It was found that silicon nitride coating structural properties are not dominant in controlling grain size or orientation. It is suggested that the particle size in the coating itself is too small to influence nucleation, but that Si3N4-particles precipitated from the silicon melt have both the size and lattice match which enables nucleation. Grain size and orientation are affected by cooling rate, but the growth mechanisms in the initial stages of solidification are also determining the microstructure of the ingot. These mechanisms should therefore be further investigated.