Dislocations in multicrystalline silicon
CHEETAH short on-line course
Nucleation, growth and relaxation of dislocations in directionally solidified multicrystalline silicon
|Dr.||Birgit Ryningen||9th June 2017
11:00 –12:10 CEST
Department of Energy Conversion
Multicrystalline silicon is the most promising candidate for enabling large scale production of low cost solar cells. The efficiency of such cells is fundamentally limited by the presence of crystal defects, and there is little absolute knowledge about the defect generation mechanisms, because all generation and development happens at too high temperature to be observed by conventional characterization techniques.
Crystal defects generated during the directional solidification process, particularly dislocations and sub grain boundaries, are well proven to be the factor mostly limiting the cell efficiency of multicrystalline silicon1. This means that keeping growth conditions optimal becomes extremely important. Unless the density and properties of the crystal defects are under control, any measure in other parts of the value chain, ranging from reduction of contamination, to efforts during cell processing such as gettering will have very limited effect. In contrast, recent development in crystallization process control (development of socalled High Performance Multicrystalline Silicon) has shown the large potential in efficiency improvement by reducing the amount of crystal defects.
Although directional solidification shows a large, still untapped potential and remains the most likely candidate for large scale deployment of solar energy, other silicon based technologies, e.g. deposition from the gas phase or splitting off thin layers from a single crystal may develop higher throughput combined with higher quality in the future. As with the directional solidification approach, these methods also require detailed understanding of defect generation and evolution. Understanding the fundamental characteristics of dislocation dynemics is therefore essential for these technologies as well.
In this webinar we would like to present our current understanding of dislocation nucleation, multiplication and relaxation. During growth and cool down
Post mortem characterisation (characterisation after all the dynamic processes have finished) suffers the weakness that the result being studied is a consequence of all the dynamic processes of the past; in a sense it represents the "graveyard of the dislocations". Nevertheless current understanding of microstructure evolution in silicon for solar cells is mainly based on this approach, supplemented with simulations on a global scale. This leads to controversy regarding which processes are most important in generating the observed microstructure.
|11:10-11:45||Nucleation, growth and relaxation of dislocations in directionally solidified multicrystalline silicon|
|11:45-12:00||Question & comments|