Bernhard Dipl Phys Freienstein

Fast Si sheet growth by the horizontal supported web technique

Abstract The horizontal supported web (HSW) technique was developed for the high-throughput production of sheet silicon for low-cost crystalline solar cells. The crystallization of sheet Si is brought about by pulling a carbon fibre net horizontally across the surface of an Si melt at a temperature near the melting point. The sheet grows in thickness for as long as it remains in contact with the melt, its final thickness depending on the length of the melt and the pulling speed. Pulling speeds of 1 m/min have so far been reached for sheets 6 cm in width and thicknesses between 300 and 600 It m. Electrical measurements performed on the first solar cells showed that a cell efficiency of 9% was obtained with such a supported web (SW) material.

Low Cost High Speed Si Ribbon Growth by the HSW-Technique

The development of the Horizontal Supported Web (HSW) technique for growth of Si ribbon material for solar cells has been continued. The dependence of the pulling speed on the melt length was calculated and the influence of the temperature gradient in the melt investigated. In present experiments a pulling speed of 1 m/min is used requiring a melt length of about 20 cm. At this speed the continuous growth of ribbons several meters long, 6 cm wide and 0.6 mm thick has been obtained. Test solar cells 2 cm x 2 cm in size have an efficiency of η = 11%. The cost per Watt-area were estimated for different production rates and conditions; at 6 MW/ a the cost comes down to 0.40 Dollar/Watt-area.

Fast Silicon-Sheet Growth with the Supported-Web Method

Directly grown Si ribbons or sheets are attractive for the manufacture of low-cost Si solar cells. Intensive research efforts have led to the development of ribbon growth methods that yield products suitable for the manufacture of efficient solar cells. However, the areal growth rates of most methods are rather small and this constitutes a serious obstacle to their large-scale implementation. The newly developed supported-web (S-Web) technique attempts to overcome this problem. It envisions the use of a carbon-fibre net which is pulled through a melt of liquid Si at a high pulling speed. Liquid films or webs of Si are spread out within the meshes of the net and crystallize some time after leaving the melt. In this way the formation and the crystallization of the Si ribbon is decoupled and very large areal growth rates should be possible. First experiments demonstrated the feasibility of this concept and pulling speeds of ≲ 2 m/min have been achieved. Problems exist with respect to the crystalline quality and the topography of the specimens obtained.