Photovoltaic cells comprise thin plates of semiconductor material, with the most commonly used material presently being polycrystalline silicon. The invention applies most particularly to pulling silicon ribbons for use in fabricating photovoltaic cells, with the description below thus relating to silicon, it being understood that the invention applies equally to other semiconductor materials such as germanium and type III-V semiconductor compounds of the GaAs family with congruent or quasi-congruent melting. The silicon plates are preferably obtained from a layer of silicon that forms a film deposited on a carbon substrate by pulling the substrate through a bath of molten silicon. The substrate has the form of a ribbon.
FIG. 1 is a general diagram showing the prior art method referred to as the ribbon on temporary substrate (RTS) method. A crucible 10 fitted with heater means (not shown) contains a bath 12 of molten silicon in liquid form. The bottom of the crucible has a slot 14. Using pulling means (not shown), a carbon ribbon 16 of small thickness (of the order of 200 micrometers (μm) to 350 μm) is pulled substantially vertically upwards in the direction of arrow 18 through the silicon bath 12 at substantially constant speed. The two faces 20 and 22 of the ribbon are initially covered in a thin layer of pyrolytic carbon 24 (thickness about 1 μm to 5 μm). The molten silicon wets the two faces 20 and 22 of the ribbon, and a meniscus 26 of liquid silicon forms on each face of the ribbon, with a solid-liquid connection line 28 that is situated at about 6.8 mm from the surface of the bath in the central portion of the ribbon. A thin layer of silicon 30-32 then forms on each of the two faces 20 and 22 of the carbon ribbon. The shape and the dimensions of the slot 14 are adapted firstly to allow the carbon ribbon 16 to penetrate into the crucible, and secondly to avoid molten silicon from flowing out through the slot. Although it is advantageous to obtain two silicon films 30 and 32 simultaneously, one film on each face of the ribbon, it is possible to use a technique in which only one film is obtained by preventing silicon from becoming deposited on one of the two faces.
The RTS method is described for example in patents FR 2 386 359 and FR 2 561 139.
That pulling method is nevertheless confronted with the problem of the liquid silicon meniscus being unstable in the proximity of each edge 34-36 of the carbon ribbon 16. It has been found that the solid-liquid connection line 28 tends to drop from typically about 6.8 mm to 2 mm to 4 mm relative to the surface of the silicon bath at the edges of the ribbon, over a width of about 5 mm from each edge. As a result the thickness of the silicon layer 30 or 32 that is deposited on each face of the carbon ribbon decreases going towards the edges 34 and 36 down to a value of practically zero.
FIG. 2 is a diagram showing the progressive thinning on the edges of the semiconductor layers obtained by the prior art method shown in FIG. 1. The section of the carbon ribbon 16 shown in cross-section and without the layers 24 of pyrolytic carbon is substantially rectangular in shape. The two semiconductor layers 30 and 32 are deposited simultaneously on the two faces 20 and 22 respectively of the ribbon. In the zones 38-40 and 42-44 adjacent to the two edges 34 and 36 respectively of the ribbon, the thickness of the layers decreases progressively over a distance that is typically about 5 mm. Semiconductor films made in this way are therefore particularly fragile at the edges. In addition, it is found that nucleation from grains of small dimensions propagates in the side portions of the film, thereby decreasing the photovoltaic performance of the silicon film.
Solutions to the above problem are proposed in patents FR 2 568 490 and FR 2 550 965. Those solutions consist in raising the level of the solid-liquid line at the edges of the carbon ribbon with the help of external means placed close to the edges of the ribbon. Thus, the first above-mentioned patent makes use of plates that locally raise the level of the bath of molten silicon by capillarity, and the second above-mentioned patent proposes placing a trough in register with each edge of the silicon ribbon, likewise for locally raising the level of the bath of molten silicon. Those solutions complicate fabricating the pulling structure and the pulling operation itself.