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International audience; The ribbon on sacrificial template (RST) process is a ribbon direct-wafering technology with specific ability for high throughput and thin multicrystalline wafer production, in the range of 60-140 μm. Mechanical and electrical properties of the RST material were investigated. Ball on ring and four-point bending tests showed good fracture stress values up to 260 MPa. The conversion efficiency potential for passivated emitter and rear cells (PERC) made out from the RST material, around 16%, is shown to be limited by defects reducing minority carrier lifetime. The interaction between impurities, such as C and transitions metals, with structural defects such as dislocations, results in highly recombinative areas in RST wafers. A model is proposed which shows that the carbon substrate is an important source of carbon contamination in the silicon melt during the growth of the ribbon. This high C contamination can be accompanied by transition metal contamination and can have an influence on the growth stability and on the generation of structural defects, especially if C accumulates in a boundary layer just above the growth interface. The study of the segregation of Sb indicates that the process conditions are close to the case of the diffusive regime near the solid/liquid interface, with a boundary layer thickness of about 70 μm.

Grain orientation and competition during growth has been analyzed in directionally solidified multi-crystalline silicon samples. In situ and real-time characterization of the evolution of the grain structure during growth has been performed using synchrotron X-ray imaging techniques (radiography and topography). In addition, Electron Backscattered Diffraction has been used to reveal the crystalline orientations of the grains and the twin relationships. New grains formed during growth have two main origins: random nucleation and twinning. It is demonstrated that the solidified samples are dominated by P3 twin boundaries showing that twinning on {111} facets is the dominant phenomenon. Moreover, thanks to the in situ characterization of the growth, it is shown that twins nucleate on {111} facets located at the sides of the sample and at grain boundary grooves. The occurrence of multiple P3 twins during growth prevents the initial grains from developing all along the sample, and twin boundaries with higher order coincidence site lattices can form at the encounter of two grains in twin position. The grain competition phenomenon following nucleation and twinning acts as a grain selection mechanism leading to the final grain structure.