Fraunhofer Center for Silicon Photovoltaics CSP
Fraunhofer Center for Silicon Photovoltaics CSP
Float Zone Crystals
© Fraunhofer CSP
Float Zone Crystals

High-Efficiency Silicon Float-Zone Solar-Cells

The float-zone (FZ) method allows the growth of silicon crystals with excellent properties, especially with regard to oxygen concentration. In contrast to other growth methods, the oxygen concentration is more than two orders of magnitude lower here. This makes FZ crystals particularly interesting for use in power electronics.

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© Fraunhofer CSP

Crystallization

The Fraunhofer CSP offers a platform for industrially relevant crystallizations with production-scale equipment sizes using crystallization technologies. Current focuses of our research activities include reducing process costs, optimizing crystal properties, and simplifying processes through enhanced automation.

Benefiting from the competencies of Fraunhofer CSP and its integration into the structure of the Fraunhofer Institute for Solar Energy Systems ISE, the entire spectrum of silicon technology is available following crystallization. This encompasses material characterization, crystal processing, wafer and cell manufacturing, cell certification, as well as module production and testing.

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  • Testing of silicon crystal growth to address material-specific questions:
    • Czochralski crystals up to 300 mm in diameter and 180 kg in weight: 
      • mono, multi
      • p-type, n-type
      • dopant elements (phosphorus, gallium, boron, and others) and concentration as desired
    • Float-Zone (FZ) crystals up to 4 inches in diameter
    • Silicon carbide (SiC) crystallization
    • Multicrystalline silicon - blocks up to 250 kg
  • Research and development efforts for process control and improvement
  • Production of customized crystals

Czochralski

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.

Float-Zone

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.

Multikristallizer VGF-732
© Fraunhofer CSP
The VGF-732 multicrystallizer is equipped with a G4 hot zone, which allows too produce blocks up to 250 kg.

Vertical Gradient Freeze

The Multicrystallization Plant VGF-732 is equipped with a G4 hot zone, enabling the production of blocks weighing up to 250 kg. The facility features three separately controllable heating zones, allowing for highly flexible temperature control, which is advantageous for quasi-mono crystallization and the crystallization of high-performance multi material. The "Multicrystallizer" primarily serves for the execution of specific R&D projects aimed at process optimization (temperature control, carbon reduction through gas flow and gas routing optimization). Numerical simulations based on CGSim facilitate flexible process optimization and an improved understanding of convection processes and heat flows. Other focal points include the development of new methods for phase boundary detection and in-situ measurement of crystallization velocity.

Czochralski EKZ-2700
© Fraunhofer CSP
Czochralski EKZ-2700 for the growth of mono-ingots of ≤9 " width.
  • Czochralski EKZ-270: mono-ingots of ≤9“ (length: 70 cm), p-type / n-type crystals, residual gas analyzer, feeder for Re-charging (optional)
  • 2x Czochralski EKZ-3500: mono-ingots ≤9“ (length: 200 cm), active crystal cooling,
  • Slim rod puller (DZA 3000): slim rod length: 240 cm
  • FZ-14: mono-ingots of 4“ (length: 130 cm)
  • FZ-35: mono-ingots of 8“, p-type, n-type
  • VGF-732: G4 hot-zone (250 kg), residual gas analyzer (MKS), in-situ measurement of crystallization rates
  • Vacuum induction melting furnace (Steremat)
  • String-Ribbon® Furnace »Quad«
  • Mechanical processing of feedstock: Ingot-Shaper IS-160 MK-II
  • High resolution optics for interface observation
  • GDMS (ThermoScientific): analysis of residual impurities in the ppb range, pulsed source for improved spatial resolution
  • LPS/PL: lateral photovoltage scanning with integrated photoluminescence

 

© Fraunhofer CSP

Crystallization of Float Zone Material

With the establishment of PERC cell structures and n-type mono-Si materials in silicon photovoltaics, an update of existing production lines to new manufacturing processes can be observed. The project "Cost-optimized high-efficiency solar cells made of low-oxygen n-type mono-Si silicon for industrial mass production" (KosmoS) deals with the issues associated with the development of material-compatible crystallization and sawing processes.

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Production of Mono- crystalline p- and n-Type Ingots

At present, the Czochralski method is the standard procedure for making monocrystalline ingots. Current challenges are the provision of n-type material with as little variation in resistance as possible, as the basis for highly efficient cell concepts. At the  Laboratory for Crystallization Technology we study the incorporation behavior of various doping substances and test several different ways to influence the profile of axial resistance

 

© Fraunhofer CSP

High-Efficiency Silicon Float-Zone Solar-Cells

The float-zone (FZ) method allows the growth of silicon crystals with excellent properties, especially with regard to oxygen concentration. In contrast to other growth methods, the oxygen concentration is more than two orders of magnitude lower here. This makes FZ crystals particularly interesting for use in power electronics.

More Info
 

© Fraunhofer CSP

300 mm Si-Mono-Cz Crystals

Growing 300 mm silicon mono-ingots for solar applications is challenging. The cost reduction-driven omission of an external magnet for melt convection control should not impair crystallization speed and yield. Manufacturing experiments were carried out at the Fraunhofer CSP to demonstrate the current state of process development.

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