Low-Cost Multicystalline Silicon Wafers by Purifying Metallurgical Grade Silicon with Tin Solution

The PV industry is currently facing serious difficulty in finding silicon feedstock to shore up their expansion plan to meet the rapid growth of the solar cell market. The work reported here is an effort to tackle the severe problem of silicon shortage for PV industry, and to bring down the cost for solar wafer production. A single-step process of producing silicon wafers directly from metallurgical grade silicon (MGSi) is being developed in GT Equipment Technologies, Inc (GTi). In this process, tin is used as a solvent to dissolve both silicon and impurities in the MGSi, and to retain most of the impurities during silicon crystallization. By applying the shaped crystal growth technology, silicon ribbons are pulled out of the Si-Sn melt. The thin silicon ribbons can be used directly as solar wafers. The final goal of this project is to obtain silicon ribbon that can be used directly as wafers for solar cells with targeting efficiency of no less than 14%. The preliminary objectives of the current research phase is to obtain 2" wide silicon ribbons, to analyze the purity of the grown ribbons, to characterize the electrical properties of the ribbon, and to develop a prototype puller for growing 5 to 6 inch wide silicon ribbons out of the Si-Sn solution. The experimental work on the single-step silicon wafer from MG-Si so far has demonstrated that silicon ribbons can be pulled from the MG-Si and Sn solution. Significant reduction of concentration in the grown ribbons has been observed for most of the metallic impurities. The reduction effect results from partitioning of impurities into tin. However, the reduction of both boron and phosphorus is not significant. Also, the obtained samples contain large amount of tin (several hundred ppm to 1%). The measured lifetime of the obtained wafer is still low (0.1 to 1.0 microsecond). Further reduction in boron and phosphorus level is required and the effect the tin inclusion needs to be investigated