sheets in the 50 /zm range. It is also shown that the continuous progress of silicon technology may permit the fabrication of ribbon solar cells whose performances are close to those of cells made from single-crystalline wafers. General Remarks As the thickness of an n+/p homojunction silicon cell is reduced, less photons are absorbed and more carriers recombine at the rear-side surface. This leads to reduced short-circuit current densities (Jsc) and open-circuit voltages ( Voc) and consequently to a considerable drop of the conversion efficiency of the devices. However, these effects can be obviated by means of appropriate cells structures as illustrated for Jsc in Fig. 1 [7]. Fig. 1 shows one example of the relationships between the calculated short-circuit current densities and the silicon thickness for four high- efficiency cells (HEC) against the silicon thickness. It appears in this example that two thin cells may exhibit Jsc values larger than those of thick cells; this is the case for (i) the n+/p/p+ structure noted HEC-BSF, where the p+-diffused layer generates an electric field which prevents the minority-carriers from reaching the cell back-contact, and (ii) the same structure noted HEC-BSFR, where the back surface also acts as an optical mirror which reflects long wavelength photons back into the bulk, thus increasing their absorption probability. Other more advanced thin HECs have been proposed such as the thin tandem junction cell (TTJC) [8]. All these structures lead to thin cells whose performances compare well to those of thick, conventional cells. The choice of a thickness in the 50 /zm range also follows from a trade-off between other important factors such as the practical feasibility of thin solar cell arrays, the power-to-mass ratio, Fig. 2 and the resistance to radiation, Fig. 3. In past years, thin cells have been prepared from Czochralski or Floating Zone single crystalline wafers mostly by etch-thinning techniques using alkaline or acid planar-etch solutions. The slicing of wafers 100 /zm thick has also been attempted; this technology, still in a developmental stage, may benefit from the efforts made in the same direction by terrestrial photovoltaics. A new opportunity may arise from the progress of ribbon growth techniques which can yield directly flat, high-quality sheets typically 50 to 100 /zm thick.
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