A Density Functional Study of Iron Segregation at ISFs and Σ5-(001) GBs in mc-Si Oras A Al-Ani 1,a* , J P Goss 1,b , N E B Cowern 1,c , P R Briddon 1,d , M Al-Hadidi 1,e , R Al-Hamadany 1,f and M J Rayson 2,g 1 School of Electrical and Electronic Engineering, Newcastle University, UK. 2 Department of Chemistry, University of Surrey, UK. a o.a.s.al-ani@ncl.ac.uk, b jonathan.goss@newcastle.ac.uk, c nick.cowern@newcastle.ac.uk, d patrick.briddon@ncl.ac.uk, e m.al-hadidi@newcastle.ac.uk, d r.a.s.al-hamadany@newcastle.ac.uk, g m.j.rayson@surrey.ac.uk Keywords: silicon; iron; segregation; grain boundaries Abstract Removal of the dilaterous effects of iron in silicon is critical for the performance of mul- ticrystalline silicon (mc-Si) solar cells, with internal gettering at extended defects including stacking faults and grain boundaries being one possibility. We present the results of a density function study of the behavoiur of iron at the intrinsic stacking fault and (001)–Σ5 twist grain boundary, which both represent examples of fully bonded systems. Our results show iron is bound more strongly to the grain-boundary than the stacking fault, which we ascribe to a combination of Si-Fe chemistry and strain relaxation. However, we find that the binding energy of a single Fe atom to these extended defects is modest, and less than 0.5 eV. Introduction Multi-crystalline silicon (mc-Si) based photovoltaic cells are generally accepted to currently satisfy the requirements of cost-effective large scale production techniques. However, this material has a rel- atively high concentration of point and extended defects (EDs), and both defect types are understood to act as recombination centres, limiting the operating efficiency of mc-Si solar cells [1]. Furthermore, EDs themselves are major defects affecting the stability, mechanical behaviour and transport proper- ties of materials [2], and thus the performance of solar energy application devices [3]. Iron (Fe) is a common contaminant in Si [4] having been widely investigated, and the effects of Fe in mc-Si solar cells reviewed [5]. Generally, it is agreed that Fe is especially problematic because it introduces deep levels in the band gap, reducing the minority carrier life time and becasue it diffuses very rapidly. Even in the slowly cooled directional solidification growth technique, Fe is supersaturated in the highly detrimental interstitial point defects form [6], residing at the tetrahedral interstitial site in the positively charged form (Fe + i ) in p-type Si or the neutral form (Fe 0 i ) in intrinsic and n-type Si [7]. Different processing techniques have been used in an attempt to getter Fe in order to improve the solar cell efficiency. Internal gettering by EDs including stacking faults and grain boundaries is one possibility. In particular, there is evidence that EDs interact with mobile iron defects[8]. Two exam- ples of fully four-fold bonded planar EDs in silicon are the intrinsic stacking fault (ISF) and Σ5-(001) twist grain-boundary (Σ5GB). The structures and areal energy densities of both defects have been obtained previously [9, 10] with relevant experiments [8, 11]. There is relatively little understand- ing of the mechanism of segregation of iron at these defects at an atomistic level, possibly because modelling of GBs is challenging in terms of system size required, and that the detailed structure of GBs in mc-Si remains a matter of some debate [2]. However, it is important to develop a fundamental understanding of the mechanism of any attractive interaction (gettering) between EDs in mc-Si and diffusing Fe atoms. Using first principles density-functional theory, we have modelled the energies and structures of interstitial Fe in bulk silicon and in the vicinity of and ISF and Σ5GB. The ISF is representative of an Solid State Phenomena Vol. 242 (2016) pp 224-229 Submitted: 2015-05-22 © (2016) Trans Tech Publications, Switzerland Accepted: 2015-05-26 doi:10.4028/www.scientific.net/SSP.242.224 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 128.240.229.72-10/08/15,11:03:26)