Richard Falckenberg

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.

Finite element analysis of horizontal silicon sheet growth from the melt

Abstract A mathematical analysis of the continuous horizontal growth of crystalline Si sheets from the melt is presented. The assumption is made that heat removal is by radiation from a silicon melt surface covered with a carbon fiber net. This case corresponds to the horizontal carbon-fiber supported web (HSW) technique. The mixed initial and boundary value problem of heat conduction is formulated including the solidification mechanism. A numerical solution is obtained using a finite element program. The mesh and the time step width of the program are optimized for the state at the start of crystallization. The solution yields the increase of the silicon sheet thickness as a function of position and time. The pulling speed is determined as a function of the mean temperature gradient in the melt and of the length of the melt.

Characteristics of Si ribbons grown by the S-web technique

Abstract Thin silicon ribbons have been grown with high throughput by the S-Web technique. The ribbons were up to 12 cm wide and up to 40 m long. The maximum pulling speed was 2 m/min. The crystal morphology and microelectrical properties were investigated by SEM and electron-beam-induced current. A twin structure of low electrical activity was found to be the predominant defect. Electron channeling pattern analyses have shown that a preferred orientation of the grain structure does not exist. Impurity contents were studied by infrared spectroscopy, secondary ion mass spectrometry, deep level transient spectroscopy and neutron activation analysis. The observed V-shaped impurity distribution across the ribbon thickness was explained by segregation and melt film drag out effects. Test solar cells showed efficiencies up to η = 12.0% with a mean value of ḡh ≈ 10.5%. The interaction of line and point defects with carbon and oxygen is believed to be the main factor presently limiting the solar cell efficiency. A pilot pulling unit was recently installed being capable of producing Si ribbons up to a width of 30 cm and a speed of several m/min.

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.