Solar-Grade Silicon from Metallurgical-Grade Silicon Via Iodine Chemical Vapor Transport Purification: Preprint

This conference paper describes the atmospheric-pressure in an 'open' reactor, SiI2 transfers from a hot (>1100 ..deg..C) Si source to a cooler (>750..deg..C) Si substrate and decomposes easily via 2SiI2 ..fwdarw.. Si + SiI4 with up to 5..mu..m/min deposition rate. SiI4 returns to cyclically transport more Si. When the source is metallurgical-grade Si, impurities can be effectively removed bythree mechanisms: (1) differing free energies of formation in forming silicon and impurity iodides; (2) distillation; and (3) differing standard free energies of formation during deposition. Distillation has been previously reported. Here, we focused on mechanisms (1) and (3). We made feedstock, analyzed the impurity levels, grew Czochralski single crystals, and evaluated crystal andphotovoltaic properties. Cell efficiencies of 9.5% were obtained. Incorporating distillation (step 2) should increase this to a viable level.