Coupled CVD and Float Zone Reactor for Lower Cost, Higher Efficiency Si Boules: Final Technical Report

This project modeled a new type of photovoltaic (PV) Si production reactor with the promise to reduce the cost, increase the efficiency, and reduce the energy required to manufacture Si wafers. The production of photovoltaic (PV) grade Si is well established, based on the pulling of single crystalline Si boules from polycrystalline Si created from trichlorosilane using the Siemens process. However, these processes are very energy intensive, and manufacturing of PV Si occurs almost entirely in China. To disrupt this paradigm and on-shore production of this critical material and provide benefits to the public, a substantial technological leap is required to move past incumbent technology. This study evaluated a new float zone chemical vapor deposition (Fz-CVD) concept in which trichlorosilane (TCS), the feedstock for polysilicon in the Siemens process, is injected directly into a float zone crystallizer to continuously deposit the feed rod for the boule pulling process through a combination of multiphysics modeling and technoeconomic analysis. We modeled and performed a sensitivity analysis of a hypothetical reactor in order to understand the effect of various design parameters and process flows on the operation of the reactor. Our modeling focused on two main constraints: 1) Obtaining a growth rate as high as possible for the Si feed 2) Preventing accumulation of TCS in the crystallization zone, where it could deposit on the RF heater and disrupt the crystallization process. We found that, under the right conditions, deposition rates of 100 m/min or more are possible while simultaneously keeping the crystallization zone clean. We also found that the heat input to drive crystallization could also drive the growth reaction creating a synergy by combining the polysilicon growth and boule crystallization processes. These findings informed a technoeconomic analysis of the proposed technology, which found that the Fz-CVD process consumes 6.4x less energy and creates 6.0x less carbon than the incumbent technology. Assuming a higher efficiency and service lifetime from the higher-purity float zone wafers compared to Czochralski-grown wafers, solar modules created from the CVD-Fz process offer an 8% lower levelized cost of electricity (LCOE). Therefore the technology is feasible from a technoeconomic standpoint. There are still some challenges that must be overcome to enable the technology. The technoeconomics rely on the ability to pull a 300 mm boule from a larger 450 mm seed, which has been demonstrated on smaller scales but must be established at the larger size. A pilot system is necessary to understand effects such as parasitic deposition on internal surfaces which could be a problem at the high pressures and reactant concentrations required to obtain the highest growth rates. Future research could involve a partnership between a national laboratory, a tool operator, and a potential tool manufacturer. More fundamental to increase the deposition rate of polysilicon from TCS would also be important to improving the economics of the process.