On the impact of twinning on the formation of the grain structure of multi-crystalline silicon for photovoltaic applications during directional solidification Thècle Riberi-Béridot a,n , Nathalie Mangelinck-Noël a , Amina Tandjaoui b , Guillaume Reinhart a , Bernard Billia a , Tamzin Lafford c , José Baruchel c , Laurent Barrallier d a Aix-Marseille Université, CNRS, IM2NP UMR CNRS 7334, Campus Saint Jérôme, Case 142, 13397 Marseille Cedex 20, France b Laboratoire de Mécanique de Lille (UMR CNRS 8107), Ecole Centrale de Lille, CS-20048, F-59651 Villeneuve d'Ascq Cedex, France c ESRF, 71 avenue des Martyrs, CS40220, 38043 Grenoble Cedex 9, France d Arts et Metiers ParisTech/Institut Carnot Arts Centre, Aix-en-Provence 2, cours des Arts et Métiers,13617 Aix-en-Provence Cedex 1, France article info Article history: Received 19 December 2014 Received in revised form 4 February 2015 Accepted 6 February 2015 Communicated by: Chung wen Lan Available online 16 February 2015 Keywords: A1. Directional solidification A1. X-ray topography A2. Growth from melt B2. Semiconducting silicon Electron Backscattered Diffraction Grain competition abstract Grain orientation and competition during growth has been analyzed in directionally solidified multi- crystalline silicon samples. In situ and real-time characterization of the evolution of the grain structure during growth has been performed using synchrotron X-ray imaging techniques (radiography and topography). In addition, Electron Backscattered Diffraction has been used to reveal the crystalline orientations of the grains and the twin relationships. New grains formed during growth have two main origins: random nucleation and twinning. It is demonstrated that the solidified samples are dominated by P 3 twin boundaries showing that twinning on {111} facets is the dominant phenomenon. Moreover, thanks to the in situ characterization of the growth, it is shown that twins nucleate on {111} facets located at the sides of the sample and at grain boundary grooves. The occurrence of multiple P 3 twins during growth prevents the initial grains from developing all along the sample, and twin boundaries with higher order coincidence site lattices can form at the encounter of two grains in twin position. The grain competition phenomenon following nucleation and twinning acts as a grain selection mechanism leading to the final grain structure. & 2015 Elsevier B.V. All rights reserved. 1. Introduction In the last few years, the evolution of the economic energy market has resulted in a considerable introduction of various renewable energies and technologies in daily life. The photovoltaic industry is one answer to the urgent need for renewable energies but faces competition from carbon and other renewable energies. In this context, recent work has focused on the improvement of multi- crystalline silicon (mc-Si) which presents an interesting €/Watt ratio in the production of photovoltaic panels [1,2]. However, mc-Si has an extremely heterogeneous grain structure that directly affects the solar cell efficiency via defects such as grits [3], grain bound- aries, impurity segregation [4] and dislocations [5,6]. The knowl- edge of solidification mechanisms is essential and allows control of the final grain arrangement, the occurrence of structural defects and thus the final solar cell efficiency. To unveil solidification mechanisms during silicon growth, an original directional solidification device was designed, enabling in situ and real-time characterization of the growth to be performed using synchrotron X-ray imaging techniques [7]. These techniques have been successfully used in previous studies to deepen the understanding of the grain formation mechanisms in mc-Si. Grain boundary groove dynamics have been studied and were found to play a significant role in grain selection and competition during growth [8] in agreement with existing theories [9]. The occurrence of twins has also been characterized [10]. However, a key issue remains, which is the relationship between the observed growth mechanisms, the grain orientation and the grain boundary type. Indeed, various studies show that the crystalline quality of the ingot and the twin relationship between the grain boundary types can have a significant impact on the photoelectric properties [11,12]. Moreover, although it has been shown that P 3 twins have no major impact on the photovoltaic properties, the repetition of twinning has important consequences for the final grain structure and distribution of crystallographic orientations [13,14]. The grain orientation and the grain boundary coincidence site lattice (CSL) Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth http://dx.doi.org/10.1016/j.jcrysgro.2015.02.024 0022-0248/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: thelce.riberi-beridot@im2np.fr (T. Riberi-Béridot). Journal of Crystal Growth 418 (2015) 38–44