Silicon ribbons for solar cells grown from powder by the SSP method

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

Present state of S-web SI ribbon manufacture, solar cells and modules

With the S-Web technique Si can be produced directly in form of thin plane ribbons with high throughput. In our laboratory pulling unit S-Web Si ribbons with a width of 6 cm have been grown at a speed of about 1 m/min. The efficiency of test solar cells made of such ribbons with polished surface is eta equal or smaller than 11.7%, with as grown surface equal or smaller than 11.2%. A module sized 50 cm x 50 cm made of cells 10 cm x 5 cm has an active area efficiency of eta bigger than 10% and delivers 16.2 W sub rho. Investigations indicate that the efficiency presently is limited by the carbon and oxygen contents. In a recently installed new pulling apparatus ribbons with a width of 11 cm and a length of about 5 m have been grown at a pulling speed of up to 200 cm/min. The efficiency of test solar cells is eta = 10%. Future work concentrates on the development of a continuously working pulling process in this apparatus

Manufacture of SI ribbons by the S-WEB-technique

The S-Web technique was developed for the high-throughput production sheet Si for low cost crystalline solar cells. A pilot unit was installed to demonstrate continuous Si ribbon growth. With this machine Si ribbons have been grown continuously at a speed of 1 m/min up to a length of 145 m; the width was 11-12 cm. A maximum pulling speed of 2 m/min has been achieved. Test solar cells of S-Web ribbons pulled with a laboratory unit have efficiencies up to bigger than 12%, large area standard cells have efficiencies of up to 9,8%. A module made of such standard cells had a mean cell efficiency of 9%