EPJ Photovoltaics 4, 45103 (2013) www.epj-pv.org DOI: 10.1051/epjpv/2013014 EPJ Photovoltaic s EPJ Photovoltaics O A Feasibility of using thin crystalline silicon films epitaxially grown at 165 ◦ C in solar cells: A computer simulation study S. Chakraborty 1 , R. Cariou 2 , M. Labrune 2,3 , P. Roca i Cabarrocas 2 , and P. Chatterjee 1,2, a 1 Indian Association for the Cultivation of Science, 700032 Kolkata, WB, India 2 Laboratoire de Physique des Interfaces et des Couches Minces, ´ Ecole Polytechnique, 91128 Palaiseau, France 3 Total S.A., Gas & Power – R&D Division, 92400 Courbevoie, France Received: 27 July 2012 / Accepted: 15 February 2013 Published online: 9 April 2013 c  Chakraborty et al., published by EDP Sciences, 2013 Abstract We have previously reported on the successful deposition of heterojunction solar cells whose thin intrinsic crystalline absorber layer is grown using the standard radio frequency plasma enhanced chemical vapour deposition process at 165 ◦ C on highly doped P-type (100) crystalline silicon substrates. The structure had an N-doped hydrogenated amorphous silicon emitter deposited on top of the intrinsic epitaxial silicon layer. However to form the basis of a solar cell, the epitaxial silicon film must be chiefly responsible for the photo-generated current of the structure and not the underlying crystalline silicon substrate. In this article we use detailed electrical-optical modelling to calculate the minimum thickness of the epitaxial silicon layer for this to happen. We have also investigated by modelling the influence of the a-Si:H/epitaxial-Si and epitaxial-Si/c-Si interface defects, the thickness of the epitaxial silicon layer and its volume defect density on cell performance. Finally by varying the input parameters and considering various light-trapping schemes, we show that it is possible to attain a conversion efficiency in excess of 13% using only a 5 micron thick epitaxial silicon layer. 1 Introduction The rapid growth of the photovoltaic industry and the resultant shortage of silicon feedstock supply in the last decade, prompted interest in thin crystalline wafers ob- tained either by thinning down thick ones or by devel- oping epitaxial growth processes. The latter option has gained particular importance [1–3] in view of the fact that the epitaxial silicon film can be “lifted off” from the c-Si substrate (or any other suitable substrate) on which it is grown, and be transferred to a foreign substrate [4], thus allowing for cost-saving via c-Si substrate re-use. In addi- tion, Petermann et al. [4] demonstrated that it is pos- sible to attain ∼19% efficiency in heterojunction solar cells with only a 43 micron thick intrinsic epitaxial silicon layer. However all these approaches involve temperature processes in excess of 600 ◦ C, which limits the range of suitable substrates and often require post-hydrogenation to passivate the defects in the epitaxial silicon layer. In the wafer equivalent approach used by Cariou et al. [5] the epitaxial silicon (epi-Si) films are deposited in a standard industrial radio frequency plasma en- hanced chemical vapour deposition (RF-PECVD) system a e-mail: parsathi chatterjee@yahoo.co.in on highly doped c-Si (100) substrates at 165 ◦ C, allow- ing for the additional advantage of a low thermal budget with a standard industrial tool. In these epi-Si films high crystalline quality and low stress levels have been con- firmed from HRTEM and Raman measurements. At such low growth temperatures it may seem normal for the epi- taxial film to have a high density of defects; however a high fill factor of ∼79% has been achieved in ITO/N-a-Si:H/I- epi-Si (1.7 microns)/P ++ -c-Si/Aluminium type solar cells fabricated from these epi-Si layers, testifying to the excel- lent quality of the epitaxial films produced. In fact our low temperature material comes with high hydrogen content, which provides de facto good defect passivation. Moreover since for a satisfactory diffusion length, L Eff > 3d, where d is the epitaxial layer thickness, a thinner film demands lower material quality. So far the maximum thickness of the epi-Si layer achieved, retaining good crystalline quality is ∼5 microns. However to form the basis of a solar cell, the epitax- ial silicon (epi-Si) film must be responsible for the photo- generated current of the structure. So far, in this so-called wafer equivalent approach, people made the assumption that the PV response coming from the highly doped wafer is negligible. But, to our knowledge, no quantitative study has been completed to determine whether a part of this This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.