overcome by selecting the proper characteristics of the graphite ribbon and by installing in the laboratory all the means necessary to process these ribbons up to the last step of the preparation of the ribbon substrate. 4. Layer thickness and pull rate. The RAD process has been conceived in order to achieve thin sheets at the highest pull rate possible. For ribbons 5 cm wide, an optimal compromise between the pull rate, u, and the silicon thickness, t, may be found around m=10 cm/min and t=80 /zm for the needs of terrestrial photovoltaics. In the case of space cells, where thickness control and layer quality may prevail upon the throughput rate, the criterion of fast pull rate may be somewhat relaxed. This would result in more flexible growth conditions. Two intrinsic properties of the process must be emphasized here. First, its ability to yield sheets thinner than any other process in continuous operation and second, the occurrence of an extended growth interface which permits low values of the residual stresses in the silicon sheets. 5. Preparation of unsupported sheets. The successive operations which lead from the as-grown ribbons to the unsupported silicon sheets ready for the fabrication of solar cells are displayed in the flow-chart in Fig. 10. Ribbon pieces typically 50 cm2 in size are extracted from the as-grown composite ribbon by laser cutting. The pieces thus obtained are then exposed to oxygen at high temperatures in order to burn the carbon shaper; this burn-off step is made at 1100°C for 1 hour. After burn-off, the thin silica
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