In-situ PN junction formation during edge defined film-fed growth of silicon

A novel method of forming a p-n junction during edge-defined film-fed growth (EFG) of silicon ribbons and nonagon tubes was developed using solid planar phosphorus dopant sources which were built into the crystal growth apparatus. The solid dopant sources were placed at an appropriate temperature location in the hot zone component of the growth system and adjacent to the growing crystal. The phosphorus gas evolved from the solid dopant source effectively doped the silicon crystal without contaminating the melt and a p-n junction was continuously formed on the entire length of the ribbon/tube during growth. Junction depths in the range of 750 Å to 3500 Å were obtained depending on the temperature of the source and the flow rate of the ambient argon gas in the growth furnace. Solar cells were fabricated on these wafers without the standard silicon etch and conventional diffusion steps. Ribbons where the base material was doped to 3.0 ohm-cm produced 9-11% efficiency solar cells. For nonagons, even though the sheet resistances were too high to be optimal and the base material was undoped in order to investigate back streaming of the phosphorus dopant, solar cells with efficiencies in the range of 8-8.5% resulted from this significant process simplification. This new process can be improved in the future for the current octagon growth technology and has the potential to substantially reduce the manufacturing cost of solar cells for terrestrial applications