Background

In recent years, the amount of “clean energy” solar cell production increases rapidly, encouraged by the environmental problem. Among various types of solar cell, the amount of poly-Si type increases remarkably and it is afraid that material will be short of supply due to its dependence on the semi-conductor industry. In the trend of the rise in silicon price, low cost material silicon must be continuously supplied for spreading the solar cell. Recently, metallurgical silicon refining process has been developed with the metallurgical-grade silicon as a starting material. But the high temperature refining is operated for long time in that process, therefore much lower cost refining process is expected to be developed. Regarding a new refining process at low cost, we focused on the solid solubilities of impurity elements in silicon. Solid solubilities increase in those contents once, and decrease in turn around 1473K. From this, refining at low temperature is considered to be thermodynamically effective, therefore we have tried to develop the low temperature refining process at low cost. For low temperature process, we considered the alloying of silicon and have used aluminum as an alloying element because of its especially low eutectic temperature with silicon. Therefore we investigate physical chemistry of solidification refining of silicon with Si-Al melts at low temperature. We are trying to develop the low cost refining process composed of low temperature techniques, and aiming the contribution for the moderation of an energy problem by spreading the silicon solar cell widely.

Research

Up to now, we focused on the thermodynamics of the impurity elements in solid silicon and the Si-Al melt for discussing the removal of those. In the solidification refining of silicon with the Si-Al melt, aluminum content in purified silicon ingot is considered to be the solid solubility of aluminum of the process temperature. Therefore, we determined the solid solubility of aluminum in silicon with the temperature gradient zone melting method [Pub.1]. For reducing the aluminum solubility in refined silicon, an addition of copper to the Si-Al solvent was considered as an effective way because of the strong affinity between aluminum and copper in liquid phase. Therefore, activity measurements of the liquid Si-Al-Cu system and solubility measurements of aluminum and copper in solid silicon equilibrated with the liquid Si-Al-Cu alloy [Pub2] were investigated. It was found that an addition of Cu to the melt decreases the aluminum content in solid silicon effectively, and Cu contamination is not sufficient in solid silicon. And for discussing the removal of phosphorus in this refining process, distribution ratio of phosphorus between solid silicon and Si-Al melt was investigated at the temperature range of 1173～1373K [Pub.3]. Distribution ratio was obtained as smaller than 0.1 at each temperature, which is smaller than the segregation coefficient of phosphorus in silicon, 0.35. Accordingly, it is found to be possible to decrease phosphorus by one order in this refining process. Now we are investigating the removal of boron..

Future work

The directional solidification of Si-Al melt will be operated as the trial of solidification refining of silicon.