This Small Business Innovation Research (SBIR) Phase I project aims to develop a design concept and demonstrate the feasibility of a continuous process to produce low-cost silicon wafers for solar cells. The approach is to cool a molten bath of silicon from the top to be below its melting point, producing a thin silicon sheet directly from the molten silicon bath. This technique allows the monocrystalline or highly textured silicon sheets to be continuously produced. A liquid or solid layer will be used to support the silicon and ensure uniformity.
The broader/commercial impact of this project will be the potential to significantly reduce the cost of silicon wafers for solar cells. Today, the cost of silicon wafers is about 45% of the total cost of a complete solar panel. This process is expected to reduce the cost of silicon wafer by 40-65%, and thus, greatly reduce overall cost per watt of electricity generated from solar cells.
The objective of this SBIR Phase I project was to demonstrate the technical feasibility and commercial viability of the Horizontal Ribbon Growth (HRG) process for the continuous production of silicon wafers for solar cells. Our technology will significantly reduce costs compared to the existing process for manufacturing silicon solar cells while maintaining high efficiency. The cost per watt will go down to $0.55/watt for a complete panel initially and further down to $0.40/watt in two years. The Horizontal Ribbon Growth (HRG) process is motivated by how ice freezes on water. A molten bath of silicon is cooled from the top, below its freezing point. Solid silicon is less dense than liquid silicon (by about 4%) so that the sheet floats like ice on water. The solid sheet is then continuously removed, while liquid silicon is continuously replenished at the same rate. The continuous process promises to reduce the cost of mono-crystalline silicon wafers by a factor of 3 or more relative to the current technology. The main savings come from reducing waste by eliminating sawing (kerf losses) and by continuous operation at high production rate. The production rate of the HRG process is not limited by the rate of solidification since solidification is perpendicular to the process flow. This property distinguishes the process from many current technologies including the Czochralski and Edge Film Defined Growth (EFG) processes. Furthermore, directional solidification on a liquid substrate produces a mono-crystalline sheet of high quality by moving impurities to the melt. Following are some of the milestones achieved during the SBIR Program supported by the National Science Foundation: We completed the design of an extractor table including an automatic pulling system to pull a thin silicon sheet from the molten silicon substrate. The system pulls the thin silicon sheet at different and precisely controllable speeds from the silicon melt. Using the extractor system, we were able to pull a mono-crystalline sheet of Silicon upto a depth of 300 μm by controlling the thermal profile and the pulling rate. We developed a computational fluid dynamics model and a full-scale water model to estimate the heat flow from the silicon melt to the cooling medium through solid silicon film. Our models showed that at a particular temperature, silicon sheet solidifies on top of the molten substrate, resulting in the formation of a sheet of approximately 180 μm with a pulling rate of 10-20 cm/min. We also showed that the impurities would move to the bottom, thereby increasing the purity of the silicon sheet formed on top of the substrate. We completed the design of a Pilot plant system for process verification which includes testing of the extractor system mentioned above. The plant design consists of different zones: the melt zone, stabilization zone and the extraction zone for the extraction of thin Silicon sheet. Based on the above milestones achieved in this Phase I project, we also verified that the proposed approach holds the potential to offer the following economic benefits over conventional wafering processes: Purification and silicon sheet production are carried out during crystallization. This allows the use of less clean raw materials than classical casting processes. This is an additional cost reduction opportunity. The direction of the production flow is perpendicular to the direction of crystallization. High process utilization can be expected. Production cost can be decreased significantly. The process produces thin sheets of silicon continuously and hence there are no kerf losses. This results in a major reduction in silicon loss. The design is simple and hence the capital cost will be relatively less. Several papers, conference proceedings and one patent application disclose various aspects of the proposed process. We have been able to install a unique pilot plant system at Industrial Learning Systems facilities in Pittsburgh that we will use for further studies of the HRG process. The system will be operational in the summer of 2013. It represents an investment of about $1M. We expect to be able to verify the complete process within a time frame of 3 years.
