Thin Solid Films 403 – 404 (2002) 229–237 0040-6090/02/$ - see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0040-6090 Ž 01 . 01559-0 Thin-film polysilicon solar cells on foreign substrates using direct thermal CVD: material and solar cell design G. Beaucarne *, S. Bourdais , A. Slaoui , J. Poortmans a, b b a IMEC vzw., Kapeldreef 75, B-3001 Leuven, Belgium a CNRS-PHASE, 23 rue du Loess, F-67037 Strasbourg, France b Abstract The method to reach low-cost thin-film solar cells studied here involves the deposition of a thin layer of crystalline Si directly on cheap foreign substrates with CVD at high temperatures. Our investigation deals with controlling physical phenomena such as nucleation, layer growth and dopant diffusion to achieve specific properties enabling better performance, effectively designing both the polysilicon material and the device structure. The required grain size is obtained by depositing small isolated crystallites with the right surface density on the substrate and subsequently growing a closed layer from these nuclei. To reduce recombination velocity at grain boundaries the effect of the growth rate and post-deposition treatments are investigated. The high second diode current that usually plagues such devices can be reduced by improving the material quality and optimising the base doping level. We introduce the TREBLE concept in which carrier collection relies on the presence of deep peaks of preferential doping at grain boundaries.  2002 Elsevier Science B.V. All rights reserved. Keywords: Polycrystalline silicon; Thin film Si; Foreign substrate; CVD; Solar cell; Grain boundary 1. Introduction Many in the photovoltaic industry regard thin layers of crystalline Si on low cost substrate as a promising alternative to standard bulk Si wafers. No approach among the various methods of producing such layers has yet emerged as the ultimate solution. As a result, a broad range of techniques and approaches are being investigated. Recently, techniques involving the transfer of a thin layer of monocrystalline Si onto a cheap substrate have received a lot of attention. Thanks to the good material quality of the active layer, it is expected that high efficiencies will be reached relatively easily with such procedures. A conversion efficiency of 15.3% has already been reported w1x. All other approaches to thin film crystalline Si solar cells involve a deposition process, sometimes combined with a crystallisation proc- * Corresponding author. University of New South Wales, Centre for Third-Generation Photovoltaics, Sydney, New South Wales, Aus- tralia; Tel.: q61-2-9385-4054; fax: q61-2-9385-5412. E-mail address: g.beaucarne@unsw.edu.au (G. Beaucarne). ess. A convenient way of giving an overview is to classify the approaches according to the highest temper- ature of the whole process sequence, which is usually the deposition temperature. At the lower end of the spectrum, there is the work done on nano- or micro- crystalline Si on glass, which is deposited by plasma- enhanced chemical vapour deposition (PECVD) at a temperature in the order of 2008C. Considering the low temperatures involved, very good efficiencies ()7%) have been reached. Increasing the temperature (up to 6508C), one finds a variety of techniques that still allow the use of glass and often involve some kind of low temperature crystallisation (solid phase crystallisation or metal-induced crystallisation). Increasing the tempera- ture higher into the range 700–13008C imposes the choice of another type of substrate, typically made of a ceramic material. These techniques, which have been much less intensely investigated than others, involve the direct deposition of the silicon layer with solution growth w2x or chemical vapour deposition (CVD) w3x. The research described in this paper is relevant to one of these intermediate techniques, namely the CVD route.