Kouji Yasuda and Toru H. Okabe
Institute of Industrial Science, the University of Tokyo, Tokyo 153-8505
High purity silicon used for photovoltaic application, namely solar grade silicon (SOG-Si), has been commercially supplied mainly from the off-grade high purity silicon manufactured by the Siemens process. However, recent and rapid growth in solar cell production induces a serious shortage of SOG-Si. With an aim of resolving low productivity of the Siemens process, various types of SOG-Si production/purification processes have been invented as a post-Siemens process. Some processes are currently under development aiming at establishment of commercial process. These processes can be classified into the following three technologies: (1) decomposition and/or hydrogen reduction of silane gases by improving the current commercial Siemens-based processes, (2) metallothermic reduction of silicon halide compounds by zinc or aluminum, and (3) upgrading metallurgical-grade silicon by employing metallurgical purification methods.
This paper reviews the features of the SOG-Si production processes, particularly the processes based on the metallothermic reduction. These metallothermic reduction processes are classified from the viewpoint of the reductant types and silicon compounds. The future prospect for the development of new high purity silicon production process is presented.
high purity silicon, solar cells, polycrystalline silicon, smelting, reduction process, metallothermic reduction
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