PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS Prog. Photovolt: Res. Appl. 2005; 13:201–208 Published online 18 February 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pip.579 SHORT COMMUNICATION Thin-film Free-standing Monocrystalline Si Solar Cells with Heterojunction Emitter C. S. Solanki 1,3 , L. Carnel 1 , K. Van Nieuwenhuysen 1 , A. Ulyashin 2 , N. Posthuma 1 , G. Beaucarne 1, * ,y and J. Poortmans 1 1 IMEC vzw, Kapeldreef 75, 3001 Leuven, Belgium 2 Fernuniversita ¨t Hagen, 58084 Hagen, Germany 3 IIT, Energy Systems Engineering, Powai, Mumbai-76, India We propose a novel approach to thin-film silicon solar cells, namely the freestanding monocrystalline silicon layer transfer process with heterojunction emitter (FMS-HJ). High crystallographic quality mono-Si films were deposited on freestanding porous silicon (PS) films by chemical vapor deposition (CVD). These free-standing mono-Si (FMS) films were processed into solar cells by creating a-a-Si/c-Si heterojunction. In our preliminary experiments a thin-film FMS-HJ solar cell with 96% efficiency was realized in a 20-m-thin active layer. Copyright # 2005 John Wiley & Sons, Ltd. key words: Si; solar cells; thin-film; free-standing; porous silicon; monocrystalline films; heterojunction INTRODUCTION C rystalline silicon (c-Si) is an excellent material for solar cells. It provides high and stable cell efficien- cies. It is non-toxic and the raw material is abundantly available in nature. The fabrication of solar cells based on thin c-Si films on foreign substrates appears to be one of the most attractive routes to realize cheap and efficient solar cells. 1–4 There are many different approaches to thin-film c-Si solar cells. Most of them yield a material with a high density of crystallographic defects, and lead to substantially lower performance than traditional bulk crystalline Si solar cells. There are however thin-film approaches that lead to monocrystalline silicon films, so-called layer transfer processes (LTP). 4–7 The best thin-film Si solar cell efficiencies so far have been obtained with LTPs. We have recently proposed a new type of LTP, the free-standing monocrystalline sili- con (FMS) film solar cell process. 8 Like several other LTP’s, this process is based on porous silicon (PS). The PS is obtained by anodization of Si in an HF solution. 9,10 However, instead of leaving the PS film on the starting wafer as in other LTPs, the PS film is separated from it at the end of the electrochemical etching step. An inno- vative procedure that achieves the formation and automatic separation of such PS film with one single anodiza- tion step has been developed for that purpose. 11 The PS film is then transferred to a CVD reactor and used as a template for the epitaxial growth of an active Si layer. High-quality monocrystalline epitaxial layers have been obtained with this procedure. The process makes it possible to reuse the expensive Si substrate. Reusability is a Received 13 April 2004 Copyright # 2005 John Wiley & Sons, Ltd. Revised 28 June 2004 * Correspondence to: G. Beaucarne, IMEC vzw, Kapeldreef 75, 3001 Leuven, Belgium. y E-mail: beaucar@imec.be Contract/grant sponsor: European Commission; contract/grant number: NNE5-2001-00628. Research