Crystallization

The Fraunhofer CSP operates modern and high-performance crystallization systems for producing single crystals up to 300 mm in diameter, including one EKZ-2700 and two EKZ-3500 units (all PVA-Tepla). These facilities are equipped with optimized hot-zone designs, graphite funnels, and separately ventilated locks, enabling the crystallization of multiple crystals from a single crucible. The process is largely automated, with various recipes available for diameter variation, shoulder tilt adjustment, or shortened end cone formation. Additional features include a recharging setup for maximizing crucible load or for experiments with multi-pulling, as well as active crystal cooling to enhance heat dissipation during crystallization, allowing for faster crystallization rates.

Current research focuses on extending crucible lifespan, oxygen transport into the growing crystal, and process optimization for time and energy management. Furthermore, test crystallizations are conducted for polysilicon manufacturers to address issues related to material quality, resulting resistance distribution, carrier lifetime, and ultimately the resulting solar cell efficiencies.

With the FZ-14, Fraunhofer CSP has the capability to grow Float-Zone (FZ) crystals up to a diameter of 4" (100 mm) and a length of up to 130 cm. The focus of Float-Zone activities includes material testing to assess the suitability of polysilicon rods for FZ applications, the production of stock rods from cost-effective solar-grade silicon, and enhancements in process automation. The advantages of this technology, such as faster crystallization rates, low oxygen and carbon concentrations, and elimination of crucible costs, hold significant potential for application in the photovoltaic sector.

FZ crystals generally exhibit the highest carrier lifetimes (> 6-8 ms), resulting in the highest cell efficiencies. Prerequisites for broader application include a higher degree of process automation and ensuring an adequate supply of FZ-compatible raw materials. The ability for gas-phase doping enables the establishment of very homogeneous dopant profiles, which is an interesting and important aspect concerning the crystallization of n-type silicon.