Development of High Efficient Production Process of Materials for Solar Cells

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In order to develop a new high efficient production process of silicon for solar cells by refining inexpensive metallurgical grade silicon, following experiments were conducted.1. The thermodynamic data (the activity coefficients, the interaction parameters and so on) related to impurities such as boron, aluminum, calcium, oxygen and nitrogen in molten silicon at 1723K and 1773K were measured by using the chemical equilibration techniques.2. Vacuum refining under the atmosphere of 10 Pa was applied for removing phosphorus, calcium and aluminum from molten silicon by using vacuum treatment was also examined from the thermodynamic viewpoint.3. Slag treatments were applied for removing boron and phosphorus from molten silicon.(1) The distribution ratios of boron and phosphorus between slags and molten silicon which are defined as :Lx ≡(mass% of X in slag)/[mass% of X in molten silicon], X = B or Pwere measured under an inert gas atmosphere at 1723 K and 1773 K and the following results were obtained :・ High value of distribution ratio of boron is not achieved even by using the NaOィイD20.5ィエD2-CaO-SiOィイD22ィエD2 slag saturated with SiOィイD22ィエD2, because high basically and high oxygen potential can not be simultaneously achieved in this process.・ High value of distribution ratio of phosphorus between the CaO-CaFィイD22ィエD2 slag saturated with CaO and molten silicon is achieved under the condition of lower oxygen potential.(2) The CaO-CaFィイD22ィエD2 slag addition-oxidizing gas injection method was applied for removing boron from molten silicon at 1773 K. Nearly 85 % of boron could be rapidly removed from molten silicon.