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Silicon sheets for solar cells grown from silicon powder by the SSP-technique
Silizium-Platten für Solarzellen, hergestellt aus Silizium-Pulver nach der SSP-Technik
A thin layer of silicon powder is poured onto supporting plates made of quartz. In a second step the surface or this layer is melted by focussing light of halogen lamps onto the surface. The liquid silicon combines the powder particles. Thus a self-supporting pre-sheet is formed which can easily be removed from the quartz plates. In a third step this pre-sheet is converted into the final coarse-grained sheet by zone melting also using focussed light as heat source. The feasibility of the technique was demonstrated (1,2) by fabricating sheets of up to 80 x 150 qmm at a thickness of only 350 mym. Now, a new machine has been built up which allows continuous processing of ribbons of 100 mm width. The same sequence of process steps and the same heating technique is used. Starting material is a mixture of undoped and highly doped silicon granulate grown from a CVD process in a fluidized bed reactor. The sheets are characterized by metallographical and electrical methods. Grains extendto som e cm in length and some mm in width showing a predominance of <211> in growth direction. Strong variations in the etch-pit-density (10 high 3-10 high 6/qcm) were correlated with variations in EBIC contrast. Test solar cells (20 x 20 qmm) from these demonstrated already the quality of the SSP-technique. Conversion efficiencies ranged up to 13,2 % (I sub SC = 29.8 mA/qcm, V sub OC = 563 mV, FF = 0.78). From I sub SC decay measurements local diffusion lengths of up to 250 mym were calculated.