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Experimental study of the grain boundary orientations in multi-crystalline silicon

Thierry Duffar 1,  C. T. Nwosu 1,  I. M. Asuo 1,  J. Muzy 1,  Y. Duterrail-Couvat 1,  F. Robaut 2

1. SIMaP EPM, CNRS UJF, BP 75, Saint Martin d'Heres cedex 38402, France
2. INP-Grenoble, BP 75, Saint Martin d'Heres cedex 38402, France

Abstract
There are numerous Grain boundaries (GB) in photovoltaic multicrystalline silicon, with the drawback of decreasing the solar cell efficiency.  We developed a theoretical model [1] in order to describe the origin and development of GB during the solidification of the silicon ingot. This model is used in a numerical code for the simulation of the GB distribution in industrial ingots [2]. One of the conclusions of the theoretical model is that facets may exist at the liquid-grain-grain triple line during growth. In this case, the orientation of the grain boundary is predicted to follow the bisector of the groove formed by the two facets.

The objective of this work is to validate experimentally this theoretical expectation. In this purpose, appropriate zones in Si ingots were selected, cut and polished at places showing interesting GB configurations with respect to the model hypothesis.  These samples were analyzed by EBSD with a SEM in order to find the crystallographic orientation of the two grains forming the GB. Then the shape of the GB was analyzed by successive polishing and 3D reconstruction from successive pictures.

In the case where the crystallographic orientation of the two grains is compatible with the existence of (111) facets at the triple line, it was found that straight grain boundaries are always obtained. Comparison of the grain boundary orientation with the orientation of possible facets shows, within the error margin, that the GB follows the bisector of the two facets.

[1] « The solid-solid-liquid triple line and its effect on the grain structure of multi-crystalline photovoltaic silicon” T. Duffar and A. Nadri, C. R. Ac. Sci. Ser. Physique, 14 (2013) 185–191.

[2] “ Numerical simulation of grain structure in PV Si” A. Nadri, Y. Du Terrail, T. Duffar, IWMCG7, 28-31 October 2012, Taipei, Taiwan, submitted for publication in the J. Crystal Growth.

 

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Presentation: Oral at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, General Session 7, by Thierry Duffar
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17

Submitted: 2013-04-12 18:46
Revised:   2013-07-23 16:30