Josef Grabmaier

Removal of C/SiC from liquid silicon by directional solidification

Abstract Silicon, produced by a carbothermic reduction of high-purity silica with high-purity carbon in an arc furnace, contains a high amount of carbon and silicon carbide. For this reason, a subsequent purification step is required to obtain solar grade silicon for the production of solar cells with high efficiencies. The investigation of a directional solidification process for that purpose is described. The content of dissolved impurities is reduced by segregation, whereas a sufficient separation of silicon carbide particles can only be obtained with a convenient convectional flow in the melt, which depends on the thermal conditions during the crystallization procedure. The motion of the particles in front of the freezing interface can prevent the incorporation into the growing crystallites. This correlation is derived from experimental results and the numerical simulation of the thermal free convection in the silicon melt.

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.

Silicon sheets grown from powder layers by a zone melting process

Abstract The growth of silicon sheets from fine-grained powder by a zone melting process is reported. Incoherent focussed light is used as the heat source. Polycrystalline sheets of 100 mm × 50 mm and a thickness of about 700 μm are grown. Recent modifications of the melting process led to fairly large grain sizes. Test solar cells of 20 mm × 20 mm were fabricated and showed efficiencies of up to 11.5%.

Manufacture of Si ribbons by the S-web-technique

The S-web technique was developed for the high-throughput production of 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 >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%.<<ETX>>

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.

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

The design of a machine for semicontinuous production of 100-mm wide silicon ribbons is presented. As the grain selection takes place at the beginning of zone melting, continuous processing instead of single-sheet processing is reasonable and leads to fairly wide grains (on 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 intragrain defect density there is a significant effect of hydrogen passivation which is demonstrated by EBIC and LBIC measurements. The best solar cells made from silicon sheets from powder (SSP) material showed efficiencies above 13%.<<ETX>>

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.