Solar-grade silicon from metallurgical-grade silicon via iodine chemical vapor transport purification

In an atmospheric-pressure "open" reactor, SiI₂ transfers from a hot (>1100°C) Si source to a cooler (>750°C) Si substrate and decomposes easily via 2SiI₂ → Si + SiI₄ with up to 5μm/min deposition rate. Sil₄ returns to cyclically transport more Si. When the source is metallurgical-grade Si, impurities can be effectively removed by three 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 and photovoltaic properties. Cell efficiencies of 9.5% were obtained. Incorporating distillation (step 2) should increase this to a viable level.