Silicon ribbons for solar cells grown from powder by the SSP method
Silicium Bänder für Solarzellen hergestellt aus Pulver nach der SSP-Methode
Aim of the SSP project (Silicon Sheets from Powder) is to grow large area silicon substrates for solar cells of high conversion efficiency. A silicon powder layer is converted into a polycrystalline sheet by a zone melting process using incoherent focussed light as heat source. The feasibility of the method had been demonstrated before by the growth of 60 x 150 qmm sheets. Now the design of a new machine for semi-continous production of 100 mm wide silicon ribbons is presented. As the grain selection takes place at the beginning of zone melting, continous processing instead of single sheet processing is reasonable and leads to fairly wide grains (in the order of 10-20 mm) extending over the whole length of the ribbon. Grain structure and defects are revealed by etching techniques. Due to the high intra-grain defect density there is a significant effect of hydrogen passivation which is demonstrated by EBIC and LBIC measurements. The best solar cells made from SSP material showed effici encies above 13%. The problem of the starting material is solved now. High purity silicon particles of 150-600 mym diameter from a SiH4-CVD-process in a fluidized bed reactor are available now and are proved to be suitable for SSP.