Fraunhofer Center for Silicon Photovoltaics CSP
Fraunhofer Center for Silicon Photovoltaics CSP

300 mm Silicon Mono Czochralski Crystals: Parameters and Conditions for Crystal Growth

Growth profile for several 8-inch and 12-inch silicon Cz-crystals
© Fraunhofer CSP
Growth profile (Diameter over crystal length) for several 8-inch and 12-inch silicon Cz-crystals grown at Fraunhofer CSP. Difference in growth speed is clearly visible. 300 mm crystals need to be grown slower to reduce thermal stress.
Growth comparison of fast grown 8-inch Cz-crystal shoulder and a slowlier grown 12 inch Cz-crystal shoulder
© Fraunhofer CSP
Comparison of a fast grown 8-inch Cz-crystal shoulder (left) and a 12 inch Cz-crystal shoulder (right), that has a slower growth rate an steeper cone angle.
Crystal growth process to a stable 300 mm Cz solar crystal
© Fraunhofer CSP
Progress of developing the crystal growth process to a stable 300 mm Cz solar crystal.

Growing 300mm silicon mono ingots for solar applications poses several challenges. Due to economic reasons, a magnet is not available at the crystal puller, yet fast crystallization is required without compromising the yield. The process window for growth conditions and control parameters is narrow and requires precise examination.

Several 300mm experiments were conducted at the Fraunhofer CSP, and the results will be presented, showcasing the current state of process development. The findings reveal a significant correlation between stable growth conditions, process parameters, shoulder growth, control parameters and the process yield.

In particular, during shoulder growth, a slow increase in diameter is necessary to maintain a homogeneous heat distribution within the crystal. The heat radiation emitted over the crystallized shoulder surface during the shoulder growth process should not increase too rapidly. Excessive cooling can generate thermal stresses in the crystal, potentially leading to the generation of dislocations and subsequent structural loss. Finding a balance between an economical and stable process is the focus of our investigations.

A scientific and methodological approach can be used to optimize these parameters, which can be further addressed through 3D Multiphysics simulations.

After several test crystallizations, which served to confirm the simulated parameters and further adjustments of the system to meet the process requirements, 300 mm Czochralski monocrystalline structure crystals were successfully produced. They are n-type doped and exhibit a high charge carrier lifetime.

This allows us to proceed optimistically to the next processing steps: mechanical crystal processing, wafering of the bricks, and the subsequent cell fabrication of solar cells with highest efficiency.