Real-time investigation of structural defect formation during the directional solidification of silicon by Synchrotron X-ray imaging techniques

Start Date
17-01-2020 09:00
End Date
17-01-2020 10:00
Room 500 - 501, Central Building
Speaker's name
Speaker's institute
Norwegian University of Science and Technology, Trondheim
Contact name
Eva Jahn
Host name
Veijo Honkimaki
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Directionally solidified multi-crystalline silicon (mc-Si) solar cells offer an attractive conversion efficiency/production cost ratio. However, the inevitable generation of structural defects during the growth process limits the photovoltaic performances. It is well established that grain boundaries and dislocations have the most detrimental impact since they gather impurity atoms and act as recombination sites for carriers. Understanding the growth kinetics and defect behaviour at all process steps is of crucial importance to control their formation and improve the quality of the final material. However, it is practically impossible to decompose the involved mechanisms that occur during the solidification process, by ex-situ characterization of the cooled down crystal, since the final structure has already been determined. Our approach to tackle this challenge is to observe in-situ and dynamically the directional solidification of silicon. A crystal is melted, solidified and cooled down while the whole process is followed by Synchrotron based X-ray imaging techniques: X-ray radiography and Bragg diffraction imaging (Topography). These are the most appropriate characterization tools for the visualization and investigation of growth kinetics and lattice defects in crystals. The study is complemented by ex-situ characterisation of the samples using white beam X-ray topography, monochromatic rocking curve imaging (RCI), RCI section topography, electron back scatter diffraction (EBSD), transmission electron microscopy (TEM) among other techniques. Mechanisms, providing an understanding of the solid/liquid interface evolution, the grain nucleation and competition dynamics, the dislocation generation, multiplication and interaction between them and with grain boundaries occurring during growth and cooling down, are suggested and discussed. The impact of structural defects on the electrical behaviour of the material is also of great interest.

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