Profile
International Journal of Metallurgical & Materials Engineering Volume 1 (2015), Article ID 1:IJMME-101, 11 pages
http://dx.doi.org/10.15344/2455-2372/2015/101
Research Article
How do Impurity Inclusions Influence the Mechanical Properties of Multicrystalline Silicon?

T. Orellana1,2,*, E. M. Tejado3, C. Funke2, W. Fütterer2, S. Riepe1, H. J. Moller2, J. Y. Pastor3

1Fraunhofer Institute for Solar Energy Systems, Heidenhofstrasse 2, 79110 Freiburg, Germany
2Institute for Experimental Physics, TU Bergakademie Freiberg, Leipziger Strasse 23, D-09599 Freiberg, Germany
3Departameto de Ciencia de Materiales-CISDEM, E.T.S.I. de Caminos, Canales y Puertos Universidad Politécnica de Madrid-CSICC/ Profesor Aranguren s.n. 28040 Madrid, Spain
Dr. Teresa Orellana-perez, Institute for Experimental Physics, TU Bergakademie Freiberg, Leipziger Strasse 23, D-09599 Freiberg, Germany; E-mail: teresa.orellana-perez@physik.tu-freiberg.de
19 December 2014; 03 February 2015; 05 February 2015
Orellana T, Tejado EM, Funke C, Fütterer W, Riepe S, et al. (2015) How do Impurity Inclusions Influence the Mechanical Properties of Multicrystalline Silicon?. Int J Metall Mater Eng 1: 101. doi: http://dx.doi.org/10.15344/2455-2372/2015/101
This work was supported by the Fraunhofer Society in the frame of the project Si-Beacon, the Ministerio de Economía y Competitividad, MAT2009-13979-C03, and the Comunidad de Madrid, S-S2009/ MAT-1585-ESTRUMAT2.

Abstract

The purpose of this research is to characterise the mechanical properties of multicrystalline silicon for photovoltaic applications that was crystallised from silicon feedstock with a high content of several types of impurities.
The mechanical strength, fracture toughness and elastic modulus were measured at different positions within a multicrystalline silicon block to quantify the effect of impurity segregation on these mechanical properties. The microstructure and fracture surfaces of the samples was exhaustively analysed with a scanning electron microscope in order to correlate the values of mechanical properties with material microstructure. Fracture stresses values were treated statistically via the Weibull statistics.
The results of this research show that metals segregate to the top of the block, produce moderate microcracking and introduce high thermal stresses. Silicon oxide is produced at the bottom part of the silicon block, and its presence significantly reduces the mechanical strength and fracture toughness of multicrystalline silicon due to both thermal and elastic mismatch between silicon and the silicon oxide inclusions. Silicon carbide inclusions from the upper parts of the block increase the fracture toughness and elastic modulus of multicrystalline silicon. Additionally, the mechanical strength of multicrystalline silicon can increase when the radius of the silicon carbide inclusions is smaller than ~10 μm.
The most damaging type of impurity inclusion for the multicrystalline silicon block studied in this work was amorphous silicon oxide. The oriented precipitation of silicon oxide at grain and twin boundaries eases the formation of radial cracks between inclusions and decreases significatively the mechanical strength of multicrystalline silicon.
The second most influencing type of impurity inclusions were metals like aluminium and copper, that cause spontaneous microcracking in their surroundings after the crystallisation process, therefore reducing the mechanical response of multicrystalline silicon.
Therefore, solar cell producers should pay attention to the content of metals and oxygen within the silicon feedstock in order to produce solar cells with reliable mechanical properties.