Investigation into Efficiency-Limiting Defects in mc-Si Solar Cells Oras A. Al-Ani 1 ,a* , Ahmed M. A. Sabaawi 1,b , J. P. Goss 1 ,c , N. E. B. Cowern 1,d , P. R. Briddon 1 ,e and M. J. Rayson 2,f 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 a.m.a.sabaawi@ncl.ac.uk, c jonathan.goss@ncl.ac.uk, d nick.cowern@ncl.ac.uk, e patrick.briddon@ncl.ac.uk, f m.j.rayson@surrey.ac.uk Keywords: silicon; solar cells, interstitial iron; gettering; extended defects Abstract. First-principles quantum-chemical simulations are combined with TCAD device modelling to examine the impact of the intrinsic stacking faults and Σ 5 -(001) twist grain-boundaries on the per- formance of solar cell efficiency. We find from the combination of these computational methods, the optical properties of ideal stacking faults are similar to those of pure Si, whereas the optimised grain- boundary leads to a clear change in the real and imaginary parts of refractive index, increasing the solar-cell current density, and thus the solar cell efficiency. The impact at a device level is dependent upon the areal density of such material. So far as the optically absorption and carrier generation is concerned, segregation of diffusing iron at these planar defects has a negligible impact on device characteristics, but non-radiative recombination processes and carrier traps due to iron are expected to significantly affect efficiency in these regions. Introduction Cost reduction of photovoltaic (PV) material is an important issue in the development of future eco- nomic fabrication of solar cells. Currently, relatively expensive waferbased crystalline-Si represents the majority (more than 80%) of the PV market [1], but multicrystalline silicon (mc-Si) can offer a more cost-effective option to gain reasonable cell efficiency compared with crystalline Si [2]. The main problem with mc-Si is that it generally contains high concentrations of extended defects (EDs) and transition elements present during module fabrication [3], with iron arguably the most important impurity in lower-grade silicon [4, 5]. Interstitial iron (Fe i ) is understood to be an active recombina- tion centre, lowering device efficiency even at less than 1 ppb [6]. Furthermore, recombination, mo- bility reduction and minority carrier lifetime effects are directly associated with the both impurity and structural imperfections in the crystal structure, and hence, degrade the solar cell performance [6, 7]. However, the deleterious effects of EDs and Fe may be mitegated by gettering Fe in the form of precipitates at EDs [8, 9]. Despite numerous theoretical [10, 11] and experimental [8, 12, 13] studies reflecting the interest in the properties of EDs in Si, there is relatively limited understanding of the mechanisms for iron gettering at GBs at an atomistic level. Thus, it is crucial to develop a fundamental understanding of the mechanism of the attractive interaction (segregation) between EDs in mc-Si and diffusing Fe, and the subsequent impact upon PV efficiency. A recent study investigated the electronic and optical properties of different GBs structures on the PV performance [7], and here we present the results of a similar study to resolve the likely impact of intrinsic stacking faults (ISF) and Σ 5 -(001) twist grain-boundaries (GBs), including some reflection upon the effects of Fe i at these EDs. Methodology Different structures of doped and un-doped planar defects were modelled using density-functional the- ory, under the local-density approximation, as implemented in Ab Initio Modelling Program (AIM- PRO) [14]. Details of the simulation systems, their optimisation and energetics are provided else- where [15]. Solid State Phenomena Submitted: 2015-05-27 ISSN: 1662-9779, Vol. 242, pp 96-101 Accepted: 2015-05-28 doi:10.4028/www.scientific.net/SSP.242.96 Online: 2015-10-23 © 2016 Trans Tech Publications, Switzerland 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: 128.210.126.199, Purdue University Libraries, West Lafayette, USA-12/02/16,11:10:54)