FME-centre, work package 7 / Energy-efficient silicon production through the chemical route (2014-2017)
Solar cells demand silicon of a purity of 99.9999% or higher. The purification of so-called metallurgical silicon of 99% purity to solar or electronic grade polysilicon normally follows a production process demanding very high energy input. The industry standard for doing this, a Siemens bell reactor, gives high purity and quality, but is inefficient and consumes much energy. The project's industrial partners REC Silicon and Dynatec Engineering use other production routes, reducing the energy and cost tremendously.
The aim of this research activity, which started in 2014, is to obtain a better understanding of the complex processes involved in silane pyrolysis (thermal decomposition). It is known that the silane molecules react with other molecules to form higher order silanes, which in turn form particle nuclei. New nucleation competes with growth, and particles can form larger chains and particles by sintering. The goal is to have little waste products from this process, as well as low total energy consumption.
The industry standard is by production of trichlorosilane (TCS) and decomposition in a Siemens bell reactor, giving high purity and quality, but inefficiently and energy-consuming. Alternatively, the pyrolysis can take place in a fluid bed reactor (FBR), as used by Centre-partner REC Silicon, or centrifugal chemical vapor deposition (C-CVD) developed by Centre-partner Dynatec Engineering. The energy consumption of the latter two technologies is estimated to 5 kWh/kg or less (for the CVD step), while the Siemens method consumes well above 50 kWh/kg in the last CVD purification step.
In 2015, the research activity in the Centre has made use of a so-called free-space reactor (FSR) to conduct experiments in order to understand the silane decomposition chemistry. This reactor offers a simple geometry where we can conduct experiments under high control. The silane forms small silicon particles in the reactor hot zone. It is the preferred tool for studying the initial phases of the silicon deposition. By operating the reactor in a regime where the nanoparticles only barely have time to form, it is possible to isolate some of the complex reactions taking place in a FBR or centrifugal reactor.
This project also develops advanced monitoring tools to understand the processes and control silicon production reactors. We focus on optical monitoring of particle flow in the exhaust as well as gas analysis through gas chromatography and mass spectrometry. The tools have been specialized for silane processes, and have been developed through this and other projects at IFE.
The project also look at how we can simulate the chemistry of silane gas through numerical methods. We develop tools to simulate the reaction kinetics and thermal processes. We use computational fluid dynamics (CVD) in a program called SiSim to simulate heat transfer and gas flows in silane reactors. The SiSim program has been developed by IFE and SINTEF.
The research activity is a part of the FME centre on solar energy, a collaboration between IFE, SINTEF, NTNU and UiO, as well as 10 industrial partners. The project receives financial support from the Research Council of Norway.
Guro Marie Wyller and Thomas Preston analyzing gas measurement results from the FSR reactor.
Trygve Mongstad and Thomas Preston discuss different silicon materials.
2016-10-06 Trygve Tveiterås Mongstad / Marte Orderud Skare