Peter Dold studied Crystallography and Material Science at the Alber-Ludwigs-University in Freiburg. After finishing his PhD thesis and his Habilitation thesis (Title: ”Control of Fluid Flow and Segregation in Semiconductor Crystal Growth from the Melt”), he was awarded the Venia Legendi (the right to give lectures at the University). From 2005 to 2007, he spent two years at the Tohoku University in Sendai, Japan, performing research in the field of high-resolution in-situ observation of solidification and crystallization processes. After the stay in Japan followed a period in Kitchener-Waterloo, Canada, from 2007 to 2011 working for the Canadian company Arise. The topic was the development of a new type of polysilicon refinement process, based on a modified Siemens-Process. In 2011, Peter Dold was offered the position of Director at Fraunhofer CSP and a Professor Ship at the Martin-Luther-University Halle-Wittenberg at the faculty of physics. The main topics at Fraunhofer CSP are the ingot growth of silicon (Czochralski growth, Float-Zone growth), the wafering of silicon, and other hard and brittle materials, and the recycling of end-of-life PV-modules, LiBS batteries, and process waste.
Cz-ingot growth at Fraunhofer CSP
The progress in Czochralski growth of silicon ingots is tremendous. For many years, the standard size for solar cells was 156x156 mm², but it increased within a short time to 210x210 mm². For ingot growth, this means that the diameter increased from approx.. 218 mm to 300 mm. At the same time, the crucible charge increased to several hundred kilograms (800 kg and more are reported from China) and the operation time of crucibles in production might be as long as 300 hours. Together with the strong cost reduction required for successful operation, it results in an enormous challenge for the ingot and wafer industry.
At Fraunhofer CSP, we have three Czochralski pullers in operation, together with four multi-wire saws. We are able to grow successfully 300 mm ingots and cut 210x210 wafers using diamond wire. Our work in progress includes the increase of the hot zone dimension and the implementation of internal feeders for increased crucible filling. An important task we are working on is the reuse of the cut-off pieces and the question of dopant uniformity for gallium and for phosphorous doping.
In the second part, we will discuss the topic of recycling end-of-life PV modules or modules out of production, which did not pass the qualification test. Today, the globally installed amount of c-Si PV-systems accumulates to more than 100 million tons and just the weight of the consumed silicon exceeds more than 1.5 million tons. An industrial reliable and scalable and eco-nomically profitable recycling process for recovering silicon from end-of-life PV modules does not exist, at least not to the knowledge of the authors. We had been able to extract the cell debris from end-of-life modules, purify the silicon to a high degree, use it for Cz-ingot growth and make new PERC-cell from the wafers made out of 100% recycling silicon.
We will discuss the actual trends and developments for ingot growth and for wafering, the solved and unsolved problems as well as the questions still to be answered.