Production of Solar Grade (SoG) Silicon by Refining Liquid Metallurgical Grade (MG) Silicon: Final Report, 19 April 2001

This report summarizes the results of the developed technology for producing SoG silicon by upgrading MG silicon with a cost goal of $20/kg in large-scale production. A Heat Exchanger Method (HEM) furnace originally designed to produce multicrystalline ingots was modified to refine molten MG silicon feedstock prior to directional solidification. Based on theoretical calculations, simpleprocessing techniques, such as gas blowing through the melt, reaction with moisture, and slagging have been used to remove B from molten MG silicon. The charge size was scaled up from 1 kg to 300 kg in incremental steps and effective refining was achieved. After the refining parameters were established, improvements to increase the impurity reduction rates were emphasized. With this approach, 50kg of commercially available as-received MG silicon was processed for a refining time of about 13 hours. A half life of <2 hours was achieved, and the B concentration was reduced to 0.3 ppma and P concentration to 10 ppma from the original values of 20 to 60 ppma, and all other impurities to <0.1 ppma. Achieving <1 ppma B by this simple refining technique is a breakthrough towards the goal ofachieving low-cost SoG silicon for PV applications. While the P reduction process was being optimized, the successful B reduction process was applied to a category of electronics industry silicon scrap previously unacceptable for PV feedstock use because of its high B content (50-400 ppma). This material after refining showed that its B content was reduced by several orders of magnitude, to~1ppma (0.4 ohm-cm, or about 5x1016 cm-3). NREL's Silicon Materials Research team grew and wafered small <100> dislocation-free Czochralski (Cz) crystals from the new feedstock material for diagnostic tests of electrical properties, C and O impurity levels, and PV performance relative to similar crystals grown from EG feedstock and commercial Cz wafers. The PV conversion efficiency of 1-cm2devices made from Cz crystals grown using the new feedstock were 95% as high as those from Cz crystals grown using EG feedstock and were comparable to those we obtained using commercial <111> Cz wafers. Devices with an efficiency of 7.3% were also made directly on wafers cut from the feedstock that had not gone through a controlled directional solidification. Only a few cells have been processed. Device parameters for this material have not yet been optimized, and additional diagnostic device fabrication, analysis, and verification is under way. The successful B treatment process developed during the program can be used with high-B-doped silicon scrap from the electronics industry thereby making available, for the short term, a new silicon feedstock for an additional 200 MW/year annualproduction of PV modules. For the future, this approach, when used in an MG silicon production plant, will produce SoG silicon for $7.62/kg, which is less than the goal of $20/kg.