Growth of crystals for the photovoltaic industry
A globally increasing annual energy demand is accompanied by the growing scarcity of low-cost conventional energy sources. As a consequence, prices on international commodity exchanges are rising, as is interest in the use of renewable energy sources.
One area of renewable energy that has steadily gained importance, especially in recent years, is photovoltaics.
More than 90% of current solar cells are produced on mono- and multicrystalline silicon wafers. The basic raw material of every solar cell is therefore quartz gravel, which is available in virtually inexhaustible quantities. Before the wafers can be processed into the required solar modules in a sophisticated production process, however, these silicon slices are produced by the controlled solidification of a silicon melt. A wide variety of processes are available for crystal growth:
Monocrystalline silicon crystals
- Czochralski process
- Floating-zone process
Multicrystalline silicon crystals
- Ribbon processes, such as EFG and String Ribbon
- Bulk processes, such as the Bridgman process, VGF, VB, HEM, and block casting
to name just a few.
At the Institute of Electrotechnology, two processes are currently being investigated using numerical simulations.
In the block-casting bulk process, liquid silicon is poured into a crucible that enables directional solidification, usually from bottom to top. Non-stationary magnetic fields ensure that the silicon, while still liquid, remains in constant motion, similar to stirring a cooking pot. Under these conditions, impurities in the melt migrate toward the crucible walls, leaving the inner crystal region free of impurities.
In edge-defined film-fed growth, hollow polygonal cylinders approximately 6 to 7 m long and about 300 µm thick are drawn from a continuously replenished silicon melt. In a subsequent laser-cutting process, mechanically highly robust wafers are cut with a minimum of process waste. In this way, cost-effective multicrystalline solar modules with an efficiency of currently 14.0% can be produced.
