Multi crystalline solar cells made of silicon provide the largest market share in the photovoltaics industry. The silicon for these cells is grown by directional solidification. During the growth process, carbon monoxide from the furnace atmosphere and silicon nitride from the crucible coating are dissolved by the silicon melt in addition to the carbon already present in the silicon feedstock. At later stages of the growth the melt supersaturates with respect to carbon and nitrogen, so SiC and Si₃N₄ particles are precipitated. These particles can grow up to several hundred micrometers in diameter and lead to severe problems during the wire sawing process for wafering the ingots. Furthermore particles may act as nucleation sources for silicon grains, leading to a grit structure of small grains, or act as sources for dislocations. If the SiC is doped with nitrogen from the dissolved crucible coating, it becomes semiconducting and may act as a shunt, short circuiting parts of the solar cell. For these reasons, the incorporation of such particles needs to be avoided.
Under terrestrial conditions, the transport of particles in the melt is dominated by convection and gravitational sedimentation. During the TEXUS 51 sounding rocket mission (April 2013), a silicon crystal of 8mm diameter, with an oxide layer of approximately 5µm thickness on the surface to eliminate Marangoni convection, will be processed. Microgravity provides a purely diffusion dominated regime for the crystal growth, as shown in an older experiment on TEXUS 12, which offers the opportunity to investigate particle transport and incorporation only as a function of growth rate and particle size to serve as a baseline for ground –based processes. For a better understanding of particle behavior during the growth of solar silicon, SiC particles of variable size and shape are placed in single crystal silicon rods. This is achieved by drilling a hole of 2mm diameter, filling in the particles and closing the hole by melting the surface of the rod until a film of silicon covers the hole. The samples are processed under a vacuum of 1*10^-5 mbar or better, to prevent gas inclusions. On the ground, during the float-zone growth the particles are expected to distribute in the melt, due to thermal convection. In dependence of the growth rate, particles of a given size should be incorporated or pushed by the solid-liquid-interface. Experiments have shown that in contrast to the expectations the particles stay agglomerated in the cylindrical shape of the former depot. Furthermore these depots sink to the lower phase boundary and are incorporated at the interface for particle sizes of 60µm and larger, but get pushed along in the case of a particle size of 7µm, despite the fact that the mass is roughly the same. Pötschke [1] observed that particles in the melt coagulate if convection is present, and depending on their density, they sink or float in the melt. Despite the general idea given by this observation it is unclear why the depots behave differently if the only significant difference is the size of the particles forming them. However, further experiments have shown that either slow growth rates of 0.2mm/min or the application of a rotating magnetic field lead to the desired distribution of the particles and the depots were scattered.

[1] Pötschke, J.: Metall-Oxydispersion, TEXUS 1-10 Abschlussbericht, S.59-61

• Legal notice:
  

  Copyright (c) Pielaszek Research, all rights reserved.
  The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
  In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/29488 must be provided.

1. Formation of dislocations in highly doped n-type Czochralski silicon
2. Grain and Subgrain Boundaries in Multi-Crystalline Silicon
3. HCl assisted growth of thick 4H-SiC epilayers by chemical vapour deposition
4. Challenges in material improvement and cost reduction for crystalline silicon for PV application
5. Evaluation of different baffle geometries in an ammonothermal system by a local numerical 3D model
6. Influencing the crystallization behaviour of multi crystalline silicon in a R&D furnace
7. Interaction of Foreign phase particles with moving solid-liquid interface during directional solidification of silicon for photovoltaics
8. SiGe crystal growth in the absence of the crucible
9. Low dislocation density GaN-templates grown by the Low Pressure Solution Growth technique