Modified silicon crystals are the primary basic material used in the manufacture of solar cells for general photovoltaics (PV). However, during production of the electrical conducting crystals – a process which involves melting the raw silicon in a crucible and then cooling it in a controlled manner – metallic impurities creep in which greatly affect the performance of the cells.
In the SYNERGY joint project, researchers at Fraunhofer IISB in Erlangen, Bavaria, investigated how metallic impurities are introduced into the crystal during the process. Researchers looked at how trace metals such as iron, chromium and cobalt are penetrating the crystal, where they come from and how they can be prevented.
During transmogrification, metals accumulate in the defects of the crystal and around its edges. The researchers sought to find out where the metals were coming from – the crucible, the silicon nitride coating (which is sprayed onto its surface), or the raw material itself? In parallel to various physical experiments which targeted different metal classes, a simulation model was developed to calculate the distribution of the foreign metals in the crystal.
The results clearly showed that the crucible process was the largest source of impurities and that increasing the “purity” of materials and auxiliary instruments would significantly reduce contamination, thereby increasing crystal quality and yield. One way to improve mass production would be to create barrier layers between the crucible and silicon nitride coating, in order to block the discharge of metals from the crucible.
The results of SYNERGY will be passed on to manufacturers of PV systems and semiconductor materials. It was funded by the German Federal Ministry of Economics and Energy (BMWi) and Fraunhofer IISB worked in partnership with the AlzChem Group AG and Wacker Chemie AG.