Multicrystalline silicon is the dominant material used in photovoltaic technology for production of low-cost solar cells. Solar cells made of multicrystalline material make up a little over half of the worldwide solar cell market. The starting material is crystallised in fused silica melt containers and the obtained ingots are sliced into thin wafers to make solar cells.
Since silicon at high temperature reacts easily with almost everything, and due to purity and mechanical requirements, manufactures are restricted to very few crucibles materials. Therefore, fused silica and fused quartz glass remains the material of choice for crucible and mould applications because it is readily available in high purity form and little reactive to liquid silicon.
The three largest drawbacks of these materials are
- being a thermal insulator
- oxygen contamination and
- cost related due to single use.
Silica-based crucibles contribute with up to 30% to the conversion cost from silicon-feedstock to the as-grown ingot. Due to the difference in thermal expansion and phase transformation the crucible cracks during the cooling process. After usage, the crucible is discarded and landfilled. When projecting fused silica consumption for crucibles in line with the projected PV-growth in 2030, 3.5 Mt of silica will be consumed and landfilled.
Crucibles based on silicon nitride are potential successors to the more traditional silica-based crucibles for growth of silicon ingots. The immediate advantages of using silicon nitride crucibles are:
- their increased resilience, which enables them to be reused multiple times,
- the lower oxygen content and
- increased thermal and mechanical resistance.
Steuler Solar Technology has developed a concept for reusable crucibles based on advanced silicon nitride ceramics. The technical potential of these crucibles for crystallisation of multicrystalline silicon has been investigated at SINTEF. At the time of writing this article one silicon nitride crucible has been used for 5 subsequent crystallisation runs. The obtained silicon material quality is similar to material crystallised from standard silica crucibles. It is aimed for at least 10 times reuse. The near future plan is to scale it up to larger silicon ingots and then to manufacture solar cells and modules out of the silicon grown in this material in order to demonstrate that this technique is suitable for PV applications.
a) Standard silica crucible which breaks during the cool down, b) silicon ingot crystallised in a silicon nitride crucible and c) release of the ingot by a vacuum pad keeping the crucible intact for further runs.